Broil peppers: Preheat oven to broil. Place whole peppers onto a baking stone or sheet and broil for 5-7 minutes, turning once mid-way through broiling. Watch the peppers carefully so you don't complete char them. Remove from the heat, and place on a plate to cool.
Turn oven down: Turn oven from broil to 350°F.
Saute veggies: While waiting for the peppers to cool, make your tuna filling. Spray a skillet with cooking spray and sauté the onions and mushrooms until soft and fragrant, sprinkle with sea salt and black pepper while cooking.
Combine remaining filling ingredients: Add cooked onion and mushroom into a medium size mixing bowl. Add tuna, tomatoes, olives, cottage cheese, lemon juice, olive oil, mustard, parsley and oregano to the bowl as well. Break apart tuna with a fork and stir all the ingredients together. Season to taste with crushed red pepper and ground pepper (it should not need any additional salt).
Stuff peppers: Once the broiled peppers are cool enough to handle, carefully cut the peppers in half, remove seeds and membranes (be careful as it may still be hot inside). Lay the 4 pepper halves on the baking stone and fill with the tuna mixture. You should have just enough tuna to fill the 4 pepper halves. If you have extra, you can eat it on the side or save it for later.
Bake peppers: Place peppers in the oven for 10-15 minutes, until warm all the way through. Remove from oven, serve and enjoy.