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Here are some easy cooking tips from here.
Basic Grilled Portobello
Brush mushrooms on both sides with olive oil and season with salt and pepper to taste. Grill for 5 to 6 minutes on each side until just cooked through.
Brush Portobellos with oil. Place on a baking sheet cap sides up. Roast in a pre-heated 425 degree oven for about 20 minutes.
In a skillet, cook sliced, chopped or whole mushrooms in a little oil or butter over medium-high heat, stirring or turning until tender, about 5 to 6 minutes. Other ingredients such as onions, peppers, and garlic, can be cooked along with the mushrooms. Season with your favorite herbs or spices.
The longer you cook the portobella the meatier (firmer textured) the mushroom gets.