Whisk together stock flour, Dijon, 1 teaspoon salt, 1/2 teaspoon pepper in a 6-quart slow cooker. Heat oil in a large skillet over medium-high heat. Season brisket with salt and pepper. Cook, in batches, until browned on all side, 4 to 6 minutes. Remove to slow cooker; reserve skillet. Add coffee to reserved skillet and cook, scraping up brown bits, 30 seconds; add to slow cooker. Add potatoes, carrots, parsnips, onion, garlic, thyme, and bay leaf to slow cooker and stir to combine. Cover and cook until beef is tender, on low for 7 to 8 hours or on high for 3 to 4 hours. Discard thyme sprigs and bay leaf. Season with salt and pepper. Serve topped with thyme leaves and Pickled Red Onions, and cornbread alongside. Make the Pickled Red Onions: Stir together rice vinegar, garlic, 1/2 cup water, and 2 teaspoons kosher salt in a medium saucepan. Bring to a simmer over medium heat. Remove from heat and stir in 1/2 sliced red onion. Let stand until softened, 25 to 30 minutes.