Preheat and Prepare: Preheat your oven to 375°F (190°C). Generously grease a 9-inch deep-dish pie plate or a similar-sized square baking dish with butter or non-stick cooking spray.

Sauté the Vegetables: Heat the olive oil in a medium skillet over medium heat. Add the chopped onion and red bell pepper and sauté for 5-7 minutes, until they have softened and the onion is translucent. Add the minced garlic and cook for one more minute until fragrant. Remove the skillet from the heat and set aside to cool slightly. This step is crucial as it cooks off excess water from the vegetables and develops their sweet, savory flavors.

Prepare the Tuna: While the vegetables are cooking, thoroughly drain the canned tuna. This is the most important step to prevent a watery quiche. Press the tuna against the side of the can with a fork to squeeze out as much liquid as possible, or place it in a fine-mesh sieve and press down firmly. Transfer the drained, flaked tuna to a large mixing bowl.

Create the Egg Custard: In a separate medium bowl, whisk the 8 large eggs until the yolks and whites are completely combined and slightly frothy. Whisk in the ½ cup of milk or cream, the Dijon mustard, dried parsley, salt, black pepper, and optional cayenne pepper.

Combine All Ingredients: Add the slightly cooled sautéed vegetables to the large bowl with the flaked tuna. Add 1 cup of the shredded cheddar cheese (reserving the other ½ cup for the top). Pour the egg custard mixture over the tuna, vegetables, and cheese. Stir everything together gently with a rubber spatula until just combined.

Pour and Top: Pour the quiche mixture evenly into your prepared pie plate. Sprinkle the remaining ½ cup of shredded cheddar cheese evenly over the top.

Bake to Golden Perfection: Place the pie plate on a baking sheet to catch any potential drips. Bake in the preheated oven for 30-35 minutes, or until the center is just set (it should have a very slight jiggle, but not be liquid) and the top is golden brown and puffed. A knife inserted near the center should come out clean.

Rest Before Serving: This is a critical step! Let the quiche rest on a wire rack for at least 10-15 minutes before slicing and serving. This allows the custard to finish setting, ensuring clean, beautiful slices.