Drain canned tuna thoroughly — press firmly in a Colander or squeeze in a paper towel to remove as much liquid as possible. Excess moisture prevents browning. Mix drained tuna with mayo, lemon juice, salt, pepper, and any mix-ins like celery, pickles, or red onion. If making a sandwich melt, butter one side of each bread slice and set aside. Slice your cheese and have everything ready before the Skillet heats up — cook time moves fast.

Preheat a Skillet over medium heat for about 1 minute. Medium heat is key — too high burns the bread before the cheese melts, too low gives soggy results. For a tuna melt sandwich: Assemble with tuna on the unbuttered side of the bread, add tomato slices if using, then layer on two slices of cheese. Top with the second slice buttered side up. Place in the skillet cheese side down. Cook undisturbed 3 to 4 minutes until deep golden brown on the bottom. Flip and cook another 3 to 4 minutes until the second side is golden and cheese is fully melted. Total cook time is 4 to 8 minutes depending on pan, heat, and bread thickness. For loose tuna without bread: Add tuna mixture directly to the preheated skillet over medium heat. Stir and cook about 5 minutes until heated through and moisture has cooked off, stirring occasionally to prevent sticking.

In a small Mixing Bowl, combine the softened 3 tbsp unsalted butter softened to room temperature, minced 3 cloves garlic minced, 1 whole lemon zested and sliced into rounds zest, chopped 2 tbsp fresh parsley finely chopped, 1 tbsp fresh thyme leaves finely chopped, and 1 tbsp fresh rosemary finely chopped. Mix well until fully incorporated. Season with a pinch of 3 tbsp unsalted butter softened to room temperature and ½ tsp black pepper freshly ground.

Peel and slice onions to about 1/8-inch thick — thin enough to cook quickly, thick enough to avoid scorching. Yellow or Spanish onions work best. Limit to 2 large onions per 12-inch Skillet to prevent steaming. Have your fat ready and keep a splash of water, broth, or wine nearby to deglaze if needed.

Heat a Skillet over medium to medium-high heat. Add butter, oil, or a mix and let it shimmer. Add sliced onions in large handfuls, stirring for 1–2 minutes per batch until they begin to soften — this prevents overcrowding. Once all onions are in, season with a pinch of salt. Cook over medium heat, stirring every few minutes, for 8–12 minutes total until soft, translucent, and lightly golden. To speed up softening, add 1/4 to ½ cup water or broth and cover; steam wilts the onions in a few minutes. Remove the lid once liquid evaporates and continue cooking uncovered. If the pan looks dry, deglaze with a splash of water, broth, or wine, scraping up any browned bits. Don't crank the heat — high heat burns the outside before the onions cook through.