Clean the portobello caps with a damp paper towel, then use a small spoon to gently scrape away the dark gills on the underside. This prevents excess moisture from pooling during cooking and lets the seasonings stick. I used to skip this, and the burgers would steam instead of char.

Brush both sides of each cap with olive oil, then sprinkle smoked paprika, garlic powder, and black pepper evenly across the top and bottom. Let them sit for 5 minutes so the seasonings absorb slightly—this is why they taste seasoned all the way through instead of just on the surface.

Preheat your grill to medium-high heat, around 400°F if you're using a thermometer. This temperature high enough to get a good sear without burning the outside while the inside stays tender. Place the mushroom caps gill-side down on the grates first.

Grill for 8–10 minutes without moving them, then flip carefully and add one slice of cheddar cheese to each cap immediately. The residual heat will melt it while the second side chars slightly. I learned the hard way that flipping too early means no char marks, so resist the urge.

Grill the second side for 5–7 minutes until the cheese fully melts and pools slightly into the cap. You'll see the mushroom release some liquid at the edges—that's natural and means it's cooked through.

Toast your hamburger buns cut-side down on the grill for 1–2 minutes until they're warm and slightly golden. This gives them structure to hold up under the mushroom's weight without getting soggy.

Remove everything from heat and assemble immediately: spread a thin layer of mayo mixed with Dijon mustard on the bottom bun, layer lettuce, then the grilled portobello cap with melted cheese on top, then tomato and red onion slices, and finally the top bun.