Prep the ingredients: thinly slice the onion, shred the cheddar (if not pre-shredded), grate the Gruyère, and slice the tomato about 1/4-inch thick. Assemble the bread slices so everything is within reach. Why: mise en place keeps the quick assembly and cooking smooth and prevents overcooking the bread while you search for items.

Caramelize the onions: heat 1 tbsp olive oil in a medium nonstick or stainless steel skillet over medium-low heat. Add the sliced onion and a pinch of salt, stir to coat, then cook gently for 15–18 minutes, stirring every 2–3 minutes. If they start to stick, add a teaspoon of water to deglaze. In the last 2 minutes add a quick pinch (about 1/2 tsp) of granulated sugar to help them brown evenly. Why: low-and-slow cooking draws out onion sugars and creates deep, sweet flavor without burning — high heat would char the outside while leaving raw texture inside.

Butter and spread: soften half the butter (about 28 g) until spreadable. Spread butter evenly on the outside faces of all 4 bread slices; on the inside faces spread 1/2 tbsp Dijon mustard each on two slices. Why: spreading butter on the outside (rather than brushing only) gives an even golden crust; mustard on the interior adds acid and umami that balances the fat and sweetness.

Build the sandwiches: On the two un-buttered interior faces layer as follows for each sandwich: a thin layer (about 1 tbsp) of caramelized onions, then an even layer of shredded cheddar (about 56.5 g / half the cheddar per sandwich), scatter grated Gruyère (about 28 g per sandwich) over the cheddar for complexity, top with 3–4 slices of tomato, a few basil leaves, and a light sprinkle of salt and pepper. Close with the buttered exterior bread slice. Why: alternating cheeses (cheddar for sharpness, Gruyère for nuttiness) creates an even melt and deeper flavor; placing onions and tomato between cheeses helps insulate and prevent sogginess while they warm through.

Heat the pan and toast: place a heavy skillet or griddle over medium-low to medium heat and let it preheat for 3 minutes. Don’t use high heat — medium-low gives the cheese time to melt without burning the bread. Place sandwiches in the pan and press gently with a spatula or a weighted press for even contact. Cook 3–4 minutes per side, flipping once, until the exterior is deep golden brown and the internal cheese is fully melted. Why: slower, steady heat ensures the sandwich reaches a molten interior while developing an even, crisp crust; pressing improves contact for an evenly toasted surface.

Rest and slice: transfer sandwiches to a cutting board and let rest 1 minute, then slice on the diagonal. Why: a short rest lets the molten cheese redistribute slightly so it doesn’t gush out immediately, and the diagonal cut gives nicer presentation and easier handling.

Serve immediately, optionally with a small side of mixed greens or a lightly dressed arugula salad to contrast the richness. Why: the sandwich is best when hot and the cheeses are gooey; a bright salad cuts the richness and completes the meal.