Place your Dutch oven over medium-high heat and add the chopped bacon. Cook for about 8 minutes, stirring occasionally, until golden brown and crispy with 3 to 4 tablespoons of rendered fat in the bottom. Using a slotted spoon, transfer the bacon to a plate, leaving the fat in the pot.

Make sure beef chunks are completely dry, then season generously with salt and pepper. Working in two batches with space between pieces, sear the beef for 8 to 10 minutes, turning to develop a deep mahogany-brown crust on at least 3 sides. Transfer each batch to the plate with the bacon.

Add the diced onion to the pot and cook for 4 to 5 minutes until softened and lightly golden. Pour in the wine and use a wooden spoon to scrape up all the fond from the bottom. Let the wine simmer and reduce for 2 to 3 minutes until slightly thickened.

Return the seared beef and bacon to the pot along with the beef stock, halved potatoes, carrots, quartered mushrooms, and thyme bundle. Bring everything to a boil, then reduce the heat to low and cover. Simmer gently for 2 hours, stirring every 30 to 40 minutes, until the beef is fork-tender and almost falling apart.

Remove and discard the thyme bundle. In a small bowl, whisk the cornstarch with 3 tablespoons of cold water until completely smooth. Stir this slurry into the stew and simmer uncovered for 5 minutes, stirring occasionally, until the sauce thickens enough to coat the back of a spoon. Taste and adjust seasoning with additional salt and pepper if needed. Serve over mashed potatoes.