In a wide pan over medium heat, add olive oil or canola oil and when the oil is glossy and hot, season both sides of the meat if desired with pinches of coarse salt and pepper. Add the chuck roast to the pan and sear it browning it thoroughly on each side, about 5 or 6 minutes each side.
Add your large diced onion, carrots, celery and potatoes to the slow cooker to create a bed for the pot roast. Over this, toss in crushed cloves of garlic. Sprinkle with a dash of salt and pepper if desired. Add chuck roast meat to the slow cooker.
Toss in stewed tomatoes, splash of red wine (optional), olive oil, chili sauce, ketchup, brown sugar, mustard, Worcestershire, soy sauce, water, and beef broth. Season with 1/2 tsp of coarse sea salt, ground pepper if desired. Set slow cooker to low and cook for 8 to 10 hrs.
When Roast is finished cooking, remove meat and vegetables with tongs and set on a platter along with cooked vegetables, leaving the juices behind. Mine yielded about 2 cups.
In a pan over medium heat, melt butter and olive oil until hot but not brown, and toss in sliced baby bella mushrooms. Season mushrooms with a pinch of salt and pepper if desired or leave plain. Brown muchrooms until golden, about 5 or 6 minutes.
Add reserved juices from the pot roast (mine yielded about 2 cups of juice from the cooker) to the pan with the mushrooms. While the juices and mushrooms take a minute to come to a simmer, combine flour and olive oil in a bowl and stir until it forms a paste. Add the paste to the pan and whisk into the juices with the mushrooms until thickened and coating the spoon. Serve slow cooker pot rost over rice or mashed potatoes and top with mushroom sauce.