Preheat the oven to 450F.
Sauté the bacon in a drizzle of oil in a heavy skillet set over medium-high heat until cooked through; transfer to a bowl (or the insert of your slow cooker) and set aside. If you like, pour out the bacon fat and add another drizzle of oil to the skillet.
Dry the beef with paper towels (it won't brown if it's damp) and brown it in batches, browning on all sides - don't worry about cooking the meat through. Add it to the bowl of bacon.
Brown the onion in the skillet, and then either transfer to a baking dish or the insert of your slow cooker; add the beef and bacon and sprinkle the lot with the flour. Set the dish uncovered in the oven for 4 minutes - remove and toss the meat and put it back in for 4 minutes more. (This browns the flour and covers the meat with a light crust.) Remove from the oven and turn the temperature down to 325F; if you're using your slow cooker, you can turn the oven off.
Stir in the wine and enough stock or bouillon so that the meat is barely covered. (I like to pour some of the liquid into the pan first to loosen all the delicious brown bits, then pour it back over the meat.) Add the tomato paste, garlic and thyme. If you're using a slow cooker, set it on low for 6-8 hours. Otherwise, cover the baking dish and set in the lower third of your oven and bake for 2 1/2 to 3 hours. The meat is done when a fork pierces it easily.
When the meat is tender, pour the contents of the baking dish into a sieve set over a saucepan; return the beef and bacon to the dish. Heat a drizzle of oil with the butter in a skillet set over medium-high heat and brown the onions and mushrooms until they turn golden. Add to the beef.
Skim any excess fat off the surface of the sauce and simmer for a minute or two, until slightly thickened. You should have about 2 1/2 cups of sauce thick enough to lightly coat a spoon. If it's too thin, cook it down a little longer; if it's too thick, add a bit of stock or water to thin it. Taste and adjust the seasoning, then pour over the meat and vegetables.
Serve immediately over mashed potatoes or buttered noodles.