To prepare the peppers, slice the tops off and remove any veins or seeds inside. Do not discard tops as you will use any remaining pepper attached to the stem.
Add peppers to a casserole dish, fill with about 1/2 inch of water, cover with aluminum foil, and bake in a 425 F oven for 20 minutes in order to steam, or parbake, the peppers.
While the peppers are steaming, cook the ground turkey, onion, garlic, and any remaining pepper from the stem piece (diced) in a large saute pan over medium high heat until the meat is crumbly, about 10-15 minutes. Add the diced zucchini and allow to cook for 1-2 minutes. Stir in the tomatoes. Add salt to taste. Remove from heat.
Once peppers are done, remove from oven, discard water, and return peppers to casserole dish. Fill each pepper cavity with the ground turkey filling until all of the peppers have been evenly filled. Top with pepper jack cheese. If making ahead, store in airtight container in the refrigerator until ready to heat and serve.
Return peppers to oven and cook until heated through and cheese has melted. This should only take a few minutes.