Instant Pot Instructions:

1. Cut the roast into 8 pieces. Trim excess fat off. Sprinkle the salt and pepper over the roast and set aside.
2. Turn Instant Pot to sauté setting. When the display says HOT add in the butter and melt. Add in the leeks, celery and carrots. Saute for 5 minutes. Add in the garlic and saute for 30 seconds. Turn off the Instant Pot. 
3. Stir in the tomato paste and the vinegar well. Pour in the broth and scrape the bottom of the pot so that nothing is sticking. 
4. Add in the mushrooms, thyme and the roast. 
5. Cover Instant Pot and secure the lid. Make sure valve is set to sealing. Set the manual/pressure cook button to 45 minutes. When the time is up let the pot sit for 15 minutes and then move the valve to venting. Remove the lid. 
6. Serve the pieces of roast with the vegetables and drizzle with the sauce. 

Slow Cooker Instructions:

1. Cut the roast into 8 pieces. Trim excess fat off. Sprinkle the salt and pepper over the roast and set aside.
2. Heat a pan over medium high heat. Add in the butter and melt. Add in the leeks, celery and carrots. Sauté for 5 minutes. Add in the garlic and sauté for 30 seconds. Add into the slow cooker. 
3. Stir in the tomato paste, vinegar, broth, mushrooms and thyme. Nestle the roast into the slow cooker. 
4. Cover and cook on low 6-8 hours. 
5. Serve the pieces of roast with the vegetables and drizzle with the sauce.