Generously season both sides of roast with salt and pepper. Sprinkle top of roast with 1 tablespoon flour and pat onto meat. Shake off excess.

Heat vegetable oil in a large skillet over medium-high heat. Sear roast until well-browned, 5 to 6 minutes per side. Remove roast from the skillet and set aside.

Reduce heat to medium. Melt butter in the skillet; cook and stir mushrooms in butter until tender, 3 to 4 minutes. Stir in onion; cook until onions are translucent and brown, about 5 minutes. Add garlic; stir until fragrant, about 1 minute.

Stir in remaining 1 1/2 tablespoons flour; cook and stir for about 1 minute. Add tomato paste and cook for another minute. Slowly add chicken broth while stirring; return to a simmer. Remove the skillet from heat.

Place carrots and celery in the slow cooker. Place roast over vegetables and pour in any accumulated juices. Add thyme and rosemary. Pour onion and mushroom mixture over the top of roast.

Cover and cook on High for 5 to 6 hours.

Skim off any fat from surface and remove bones. Season with salt and pepper.

Serve hot and enjoy!