Preheat the oven to 250°.  Heat a large, heavy-bottomed Dutch oven over medium heat.  Add the bacon and cook, stirring occasionally, until the fat is rendered and the bacon is crisp, about 10 minutes.  Remove the bacon with a slotted spoon to a paper towel-lined plate.  Pour off all but 2 tablespoons of the bacon fat to cook the beef in, reserving the remaining fat in case you need more to sear the beef.  Turn the heat to medium-high.

Pat the beef cubes dry with paper towels and season generously with salt and pepper.  In batches, taking care to keep beef in a single layer, sear the beef in the bacon fat for 4 to 6 minutes, turning to brown each side.  Remove the seared beef to a plate and continue searing until all the beef is browned.

Reduce the heat to medium.  Add the carrots (if using), onions and salt and pepper to the pan and cook for 10 to 15 minutes, until the onions are lightly browned and softened.  Add the garlic and cook for an additional 1 minute, until fragrant.  Add the Cognac and tomato paste and stir to scrape up the brown bits from the bottom of the pan.  Cook until the Cognac has reduced, about 2 minutes.  Put the meat and bacon back into the pot with all the juices that have accumulated on the plate.  Add the bottle of wine and beef stock and ensure the meat is almost covered; add additional liquid if necessary.  Add the thyme and kosher salt and pepper to taste.  Bring to a simmer, cover the pot with the lid and place in the oven for 1-1/4 to 1-1/2 hours, until the meat is very tender.  Remove from the oven and place on the stove over medium heat.

Combine 2 tablespoons of butter and the flour with a fork and stir into the stew.  Add the frozen onions.  Heat 2 tablespoons of butter to a large skillet over medium-high heat; sauté the mushrooms for 10 to 12 minutes, until softened and lightly browned.  Add to the stew.  Bring the stew to a boil , then lower the heat and simmer for an additional 15 minutes, until the liquid has thickened.  Season with salt and pepper to taste.

To serve, toast the bread slices in the oven or on a grill pan.  Rub each slice with a cut clove of garlic.  Sprinkle the stew with parsley and serve with the grilled bread.