Follow these simple steps to create delicious and satisfying Tuna Stuffed Bell Peppers:

1. Preheat Oven & Prepare Baking Dish: Preheat your oven to 375°F (190°C). Lightly grease a baking dish large enough to hold the pepper halves snugly without overcrowding. A 9×13 inch dish usually works well.
2. Prepare the Bell Peppers: Wash the bell peppers thoroughly. Slice them in half lengthwise, from stem to base. Carefully remove the seeds and white membranes from the inside. If the pepper halves don’t sit flat, you can carefully shave a very thin slice off the rounded bottom (be cautious not to cut through!). Arrange the pepper halves, cut-side up, in the prepared baking dish.
3. (Optional but Recommended) Pre-bake the Peppers: To ensure the peppers are tender, you can pre-bake them slightly. Pour about 1/4 inch of water into the bottom of the baking dish (around the peppers, not inside them). Cover the dish tightly with aluminum foil and bake for 15-20 minutes. This helps steam the peppers and softens them before adding the filling. Carefully remove the foil and discard any water from the dish after this step. Pat the insides of the peppers dry with a paper towel.
4. Sauté the Aromatics: While the peppers are pre-baking (or while the oven preheats if skipping step 3), heat the olive oil in a medium skillet over medium heat. Add the chopped onion and celery. Sauté for 5-7 minutes, stirring occasionally, until the vegetables soften and the onion becomes translucent. Add the minced garlic and cook for another minute until fragrant. Remove the skillet from the heat and let the mixture cool slightly.
5. Prepare the Tuna Filling: In a medium-sized mixing bowl, flake the thoroughly drained tuna with a fork. Ensure there are no large clumps.
6. Combine Filling Ingredients: Add the slightly cooled onion-celery mixture to the bowl with the flaked tuna. Add the mayonnaise (or Greek yogurt), Dijon mustard, fresh lemon juice, chopped fresh parsley, salt, and black pepper.
7. Mix Gently: Stir the ingredients together gently until just combined. Be careful not to overmix, as this can make the tuna filling pasty or mushy. Taste the filling and adjust seasonings (salt, pepper, lemon juice) if needed.
8. Stuff the Peppers: Carefully spoon the tuna mixture evenly into each bell pepper half. Press the filling down gently to fill the cavity completely. Don’t pack it too tightly, but ensure each pepper is well-filled.
9. Add Toppings: Sprinkle the shredded cheese evenly over the top of the tuna filling in each pepper half. If using breadcrumbs for a crispy topping, mix them with a teaspoon of melted butter or olive oil (optional, but helps browning) and then sprinkle them over the cheese. A light dusting of paprika can also be added here for color.
10. Bake the Stuffed Peppers: Place the baking dish (uncovered) into the preheated oven. Bake for 25-35 minutes, or until the peppers are tender (easily pierced with a fork), the filling is heated through, and the cheese is melted, bubbly, and lightly golden brown. If the topping starts to brown too quickly, you can loosely tent the dish with foil for the last 10 minutes.
11. Rest and Serve: Carefully remove the baking dish from the oven. Let the tuna stuffed bell peppers rest for 5-10 minutes before serving. This allows the filling to set slightly and the peppers to cool down just enough to handle and eat comfortably. Garnish with extra fresh parsley if desired.