Portobello Herb Butter Pork Sirloin Roast (Slow Cooker) Step-by-step instructions

Before you start

Before you start, make sure your slow cooker is clean and ready. Pat the pork roast dry with paper towels — this helps the butter and seasonings adhere. Slice the portobello mushrooms about 1/2-inch thick so they cook evenly without turning mushy.

Mince the garlic if using fresh cloves. Have the butter at room temperature or straight from the fridge — either works. No need to preheat the slow cooker; it will come to temperature gradually, which is exactly what you want for tender meat.

Instructions

Layer and Butter

1. 1Layer the Roast
   Place the pork sirloin roast in the center of a 6-quart slow cooker. Scatter the sliced portobello mushrooms over and around the roast. Sprinkle the minced garlic evenly over the mushrooms and roast.
   Look for The roast sits snugly in the pot, surrounded by mushroom slices and garlic.
   Tip No need to sear the pork first — the slow cooker does all the work.

At the end of this phase, the slow cooker contains the raw roast, mushrooms, garlic, and butter. The butter is still solid or just starting to melt. The ingredients are neatly layered, with the butter sitting on top.

There's no liquid yet — that will come as the meat and mushrooms release their juices during cooking. Everything looks simple and unassuming, but the aromas will build over the next few hours.

Slow Cook

2. 2Add the Butter
   Place the whole stick of salted butter on top of the roast. It will melt as the slow cooker heats, basting the meat and mushrooms.
   Look for The butter sits like a golden cap on the roast.
   Tip Use salted butter for the best flavor; if using unsalted, add a pinch of salt.

After 8 hours on low, the kitchen smells richly savory and buttery. The pork is visibly darker and has pulled away from the sides of the pot. The mushrooms have softened and darkened, and the liquid has turned into a bubbling, golden jus. You can see the meat is starting to shred just by touching it with a spoon. The slow cooker is simmering gently, and the aroma is irresistible.

Shred and Serve

3. 3Slow Cook Until Tender
   Cover the slow cooker and cook on low for 8 hours, or on high for 4-5 hours, until the pork is fork-tender and shreds easily. The internal temperature should reach at least 145°F (63°C) for safety, but cooking low and slow will take it well beyond that for fall-apart texture. Let it rest for 3 minutes before slicing.
   Look for The meat is deeply browned and pulls apart with a fork; the mushrooms are soft and bathed in buttery juices.
   Safe temperature 145°F (63°C) internal temperature (steaks, chops and roasts)
   Tip For the most tender result, always use the low setting — it gives the collagen time to break down.
4. 4Rest and Shred
   Turn off the slow cooker. Using tongs, transfer the roast to a cutting board and let it rest for at least 3 minutes. Then shred the pork with two forks, discarding any large fat pieces. Return the shredded meat to the slow cooker and stir it into the buttery mushroom juices.
   Look for The meat is moist and shreds into long, tender strands.
   Safe temperature 145°F (63°C) internal temperature (steaks, chops and roasts)
   Tip Resting the meat before shredding keeps the juices locked in.
5. 5Serve
   Serve the shredded pork and mushrooms over mashed potatoes or on toasted rolls, spooning extra pan juices over the top.
   Look for The pork is glossy with butter and mushroom jus.
   Tip The pan juices are liquid gold — don't skip spooning them over.

The pork shreds easily into long, juicy strands. The mushrooms are tender and coated in the buttery sauce. The liquid is thick enough to cling to the meat but still pourable. When you lift a spoonful, it's glossy and steaming. The dish is ready to serve immediately, and the flavors are deep, savory, and comforting.

Common mistakes

The most common mistake is rushing the cooking time. If you're tempted to turn the heat to high, resist — low and slow is what makes this pork so tender. Another mistake is using too little butter or skipping it altogether, which can leave the lean sirloin dry.

Don't skip the resting step after cooking; letting the meat rest before shredding keeps the juices from running out. Finally, avoid over-stirring the pot while it cooks — lifting the lid lets heat escape and extends cooking time.

How to tell it’s done

The pork is done when it is fork-tender and shreds easily with two forks. A thermometer inserted into the thickest part should read at least 145°F (63°C) for food safety, but for the best texture you'll want it to reach closer to 190-200°F (88-93°C) — that's when the collagen has fully broken down and the meat is meltingly soft.

The mushrooms will be dark, silky, and saturated with butter. The liquid in the pot will be a rich, golden-brown jus. If the meat is still tough, cook for another 30-60 minutes and check again.

• Steaks, chops and roasts: 145°F (63°C) in the thickest part, then a 3-minute rest