Season the steak generously on both sides with salt and pepper.
Sea salt and freshly ground black pepper, 1 bell pepper
Melt cooking fat in a dutch oven over medium-high heat, and brown the steaks for 2 minutes per side.
2 sirloin steaks, 2 tbsp. cooking fat
Remove the steaks and set aside.
Add the garlic, onion, celery, and bell pepper to the dutch oven and cook for 1 to 2 minutes or until soft.
1 onion, 2 garlic cloves, 1 celery stalk
Add the sliced mushrooms and diced tomatoes, stir and cook another 2 minutes.
8 to 10 mushrooms, 1 cup diced tomatoes
Bring the steaks back to the pot and pour in the beef stock.
1 cup beef stock
Sprinkle with the oregano, add the bay leaf, and season to taste.
1 bay leaf, 1 tsp. dried oregano
Cover and cook over medium heat for 30 to 45 minutes.
Remove the steaks, let rest for 5 minutes, and slice.
Serve the sliced steaks topped with the cooked vegetable sauce.