Steak Zucchini Boats

4 zucchini, sliced in half lengthwise;

1 lb thinly sliced roast beef, chopped into small pieces;

1 large yellow onion, sliced;

2 garlic cloves, minced;

6 oz. button mushrooms, sliced;

1 bell pepper, chopped;

1 tbsp. olive oil;

Cooking fat;

Sea salt and freshly ground black pepper;