Preheat the oven to 400°F. Brush both sides of the portobello caps with 1 tablespoon olive oil and season with salt and pepper. Place gill-side down on a baking sheet and roast for 10 minutes.
Meanwhile, heat the remaining 1 tablespoon olive oil in a skillet over medium heat. Add the onion and garlic and cook for 2 minutes until fragrant. Add the chopped broccoli and 2 tablespoons water, cover, and steam-cook for 5-6 minutes until bright green and tender.
Remove from heat and stir in the heavy cream and 1 cup of the shredded cheddar until melted and creamy. Season with salt, pepper, and red pepper flakes if using.
Flip the roasted mushroom caps gill-side up and divide the broccoli cheddar filling among them. Top with the remaining ½ cup cheddar.
Return to the oven and bake for 10-12 minutes, until the cheese is melted and golden and the mushrooms are fully tender.
Let rest for a couple of minutes before serving, so the filling sets slightly.
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