Combine broth, flour, tomato paste, 1 1/4 teaspoons kosher salt and 1/4 teaspoon black pepper in a blender and blend until smooth. Heat a large skillet oven over high heat, spray with oil and brown the beef on all sides in two batches. Transfer the beef to the slow cooker, then spray the skillet again. Add the carrots, celery, onions and garlic and brown, stirring about 5 minutes. Transfer to the slow cooker and add the potatoes, mushrooms, thyme and bay leaves and pour the broth mixture on top. Cover and cook low 8 hours, until the meat and vegetables are tender. Discard the thyme and bay leaves, stir in the peas and let them sit 1 minute until heated through. Taste for salt and adjust as needed.