You won't miss the meat in these portobello burgers that are filled with melted cheese and toasted panko breadcrumbs for crunch. A creamy honey-garlic aioli adds another layer of flavor and doubles as a dipping sauce for kale "chips".
Make aioli - (If prepping before cooking, get oven heating and mushrooms roasting first.) Chop garlic. Whisk together garlic, mayonnaise, honey, mustard, and lemon juice. (Can be done up to 5 days ahead)
Mushrooms / Kale - Remove stems and scrape out gills from mushrooms. Remove and discard stems from kale leaves. (Can be done up to 3 days ahead)
Tomatoes - Slice tomatoes.
Make filling - Combine panko, cheese, and oregano.
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Spread mushrooms, hollow-side facing up on a sheet pan. Brush with some oil and season with some salt and pepper. Transfer to the oven and roast for 10 minutes to get them started cooking.
Meanwhile, spread kale leaves out on a second sheet pan and toss with olive oil.
Remove mushrooms from the oven and reduce oven temperature to 350F / 177C. (Note: If the mushrooms have released moisture while cooking, drain off excess water and use paper towels or dish towels to pat the mushrooms dry.)
Melt butter and stir into panko / cheese filling.
Top mushrooms with tomato slices and then with panko / cheese filling.
Transfer both sheet pans (mushrooms and kale leaves) to the oven and continue roasting until panko is golden brown and kale leaves are crisp. This should take about 6 to 8 minutes for both.
Toast hamburger buns.
Sprinkle kale leaves with some salt right out of the oven.
Spread the inside of each bun with some aioli and top with a mushroom and a leaf of kale (for some extra crunch / color). Serve the remaining kale leaves with any remaining aioli for dipping. Enjoy!