Cut the zucchini in half and using a small spoon, hollow out the center out of the halves, leaving 1/4 inch thick shell on each half, to create zucchini boats. Keep only the flesh from two zucchini and discard the rest.
Squeeze excess water out of the flesh and chop well. Set aside.
Place a large skillet over medium-high heat. Add olive oil and once the oil is hot add diced onion and garlic, stir and cook for 1 minute. Push to one side of the pan.
Add ground beef and using a wooden spoon, break it up while it cooks. Mix wit the onion and garlic and continue cooking until no longer pink. Drain grease, leaving only about 2 tablespoons in the pan.
Add the diced bell pepper, reserved diced zucchini flesh, salsa and taco seasoning. Stir well and simmer on low for about 20 minutes. Taste and adjust for salt if needed.
Preheat oven to 400 degrees F. Line a baking dish with foil, to avoid cheese sticking to it, and set aside.
Using a spoon, scoop the beef mixture into the hollowed zucchini boats. Divide the mixture equally and press firm to fit in as much as possible into the zucchini boats.
Arrange into a baking dish and top with shredded cheese.
Cover with foil and bake for 25-35 minutes until cheese is melted and zucchini is cooked through. Remove foil during last 5 minutes of cooking.
Serve as is or topped with sour cream, fresh parsley, chopped lettuce and diced tomatoes.