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1
Place portobello mushroom caps into a large resealable plastic bag and pour in balsamic vinegar. Seal the bag and shake several times to coat mushrooms with vinegar; set aside to marinate.
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2
Set an oven rack about 6 inches from broiler's heat source, and preheat oven's broiler.
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3
Line a baking sheet with aluminum foil.
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4
Cut red and yellow bell peppers in half from top to bottom; remove stems, seeds, and ribs. Place the peppers cut sides down onto the prepared baking sheet.
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5
Cook under the preheated broiler until the skin of the peppers has blackened and blistered, about 5 minutes.
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6
Place the blackened peppers into a bowl and tightly seal with plastic wrap. Allow the peppers to steam as they cool, about 20 minutes. Once cool, remove the skins and discard. Chop roasted peppers.
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7
Turn oven's temperature to 500 degrees F (260 degrees C).
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8
Remove portobello mushroom caps from bag, shake off excess vinegar, and arrange onto a baking sheet.
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9
Roast mushrooms until tender and they give off their liquid, 5 to 8 minutes. Remove mushrooms from oven and set aside. Chop mushrooms when cool enough to handle.
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10
Fill a large pot with lightly salted water and bring to a boil. Stir in fettuccine and return to a boil. Cook the pasta until tender but still slightly firm, about 8 minutes; drain.
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11
Heat olive oil and butter in a large skillet over medium heat until butter has melted; cook and stir asparagus with garlic, Italian seasoning, crushed red pepper flakes, salt, and black pepper until asparagus is tender but still bright green, about 3 minutes.
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12
Mix portobello mushrooms and roasted peppers into asparagus mixture and toss fettuccine with the vegetables until combined.
