Pan-Seared Tuna Melt with Gruyère and Caramelized Onions

By AislePrompt Editorial · Published 2026-04-26 · Updated 2026-04-26

How this recipe was made — this recipe was drafted by AislePrompt's AI-assisted pipeline from technique templates and USDA-sourced nutrition data, and published by AislePrompt Editorial — see how AislePrompt recipes are drafted and published.

Seared tuna steak meets melted Gruyère cheese and sweet caramelized onions on toasted rye bread for a decadent sandwich. This French-inspired sandwich and wrap recipe, ready in about 35 minutes, pairs fresh tuna steak, olive oil, salt for a weeknight-friendly dinner that comes together with one pan and minimal cleanup. Each serving lands at about 550 calories and feeds 1, so it slots cleanly into a weekly meal plan and pairs well with a quick salad or grain on the side. Tap "Plan a meal with the AI" below to drop this recipe into your week and send the ingredients straight to Instacart. Nutrition values are approximate, calculated from USDA FoodData Central (fdc.nal.usda.gov) — verify against ingredient labels for any health-driven dietary plan.

Prep: 20 min Cook: 15 min Serves 1 French cuisine 550 cal/serving

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Ingredients

5 oz fresh tuna steak
1 tbsp olive oil
1/2 tsp salt
1/4 tsp black pepper
1/2 medium yellow onion
1 tbsp unsalted butter
2 slices (about 2 oz) Gruyère cheese slices
2 rye bread slices
1 tbsp Dijon mustard

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Instructions

Step 1: Thinly slice 1/2 medium yellow onion and sauté in 1 tbsp unsalted butter over medium-low heat for 15 minutes, stirring occasionally, until onions are golden and caramelized.
Step 2: Heat 1 tbsp olive oil in a skillet over medium-high heat. Season 5 oz fresh tuna steak with 1/2 tsp salt and 1/4 tsp black pepper. Sear tuna for 2 minutes per side for medium-rare doneness, then remove and let rest.
Step 3: Toast 2 slices of rye bread until lightly browned. Spread 1 tbsp Dijon mustard evenly on one side of each slice.
Step 4: Thinly slice the rested tuna steak and layer it evenly on one slice of the mustard-spread bread. Top with the 2 oz Gruyère cheese slices and caramelized onions.
Step 5: Place the second slice of bread mustard-side down on top to form a sandwich. Place the sandwich back into the skillet over low heat and cover with a lid or foil. Cook for 3-4 minutes until the cheese melts and the bread crisps slightly.
Step 6: Remove from heat, slice in half, and serve immediately.