Preheat the oven to 350 degrees F (175 degrees C).
Arrange English muffin halves, cut side up, on a baking sheet. Mix together tuna, onion, and pepper in a small bowl until well combined. Divide mixture evenly onto English muffins. Place 3 jalapeño slices onto each muffin half. Top each with a Cheddar cheese slice.
Bake in the preheated oven until cheese begins to bubble, about 10 minutes.
Simple Tuna Melts · Dinner Match Lab | Dinner Match Lab