Season the beef cubes with salt and pepper, then toss them with the flour until lightly coated.
Heat the olive oil in a large skillet over medium-high heat. Brown the beef in batches until all sides are seared, then transfer it to the slow cooker.
Add the onion, carrots, garlic, and tomato paste to the same skillet. Cook for 3 minutes, stirring often, then scrape everything into the slow cooker.
Add the portobello mushrooms, red bell peppers, beef broth, and thyme to the slow cooker. Stir to combine.
Cover and cook on low for 7 to 8 hours until the beef is fork-tender and the gravy has thickened.
Taste and adjust salt and pepper before serving.