Grate or thinly slice the cheddar and gruyère so they melt evenly. Spread 1/2 teaspoon of Dijon mustard on the inside face of two bread slices. Evenly divide the cheese between those two slices (about 3 oz cheddar + 1 oz gruyère per sandwich). Sprinkle 1/2 teaspoon black pepper over the cheese layers.

Butter bread

Soften the unsalted butter if needed. Spread about 1/2 tablespoon butter on the outside face of each of the four bread slices (both top and bottom of each sandwich) so the exterior will brown in the pan.

Cook sandwiches

Heat a heavy skillet or cast-iron pan over medium-low to medium heat. When hot, place the assembled sandwiches butter-side-down in the skillet (work in batches if necessary). Cover the pan loosely with a lid or a sheet of foil to trap heat and help the cheese melt. Cook for 3–4 minutes on the first side until golden brown, then carefully flip and cook 2–4 minutes more until the second side is golden and cheese is fully melted. If cheese needs more melting, reduce heat to low, cover, and steam for another 1–2 minutes. Total cook time per sandwich: ~6–9 minutes.

Rest and slice

Remove sandwiches to a cutting board and let rest 1 minute. Slice each sandwich in half on the diagonal and keep warm while you finish the salad. If making multiple sandwiches, place on a sheet pan in a 200°F (95°C) oven to keep warm.

Side Salad

Prep vegetables

Rinse and dry the mixed greens thoroughly (use a salad spinner or towel). Halve the cherry tomatoes. Peel seeds if desired from the cucumber and slice into half-moons to total about 1/2 cup. Thinly slice the red onion to yield about 2 tablespoons.

Make dressing

In a small bowl or jar combine 3 tablespoons extra-virgin olive oil, 1 tablespoon red wine vinegar, 1 teaspoon honey, 1/2 teaspoon Dijon mustard, 1/2 teaspoon salt, and 1/4 teaspoon black pepper. Whisk or shake vigorously until emulsified and taste; adjust salt, vinegar or honey as needed.

Toss salad

Place the mixed greens, cherry tomatoes, cucumber, and red onion in a large bowl. Drizzle the dressing over the salad and toss gently until evenly coated. Taste and adjust seasoning.

To serve

Plate one grilled cheese sandwich (halved) alongside a generous portion of the dressed side salad. Add pickles or cornichons on the side if using. Serve immediately while the sandwich is hot and the salad is crisp.