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.
Set an oven rack about 6 inches from broiler's heat source, and preheat oven's broiler.
Line a baking sheet with aluminum foil.
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.
Cook under the preheated broiler until the skin of the peppers has blackened and blistered, about 5 minutes.
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.
Turn oven's temperature to 500 degrees F (260 degrees C).
Remove portobello mushroom caps from bag, shake off excess vinegar, and arrange onto a baking sheet.
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.
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.
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.
Mix portobello mushrooms and roasted peppers into asparagus mixture and toss fettuccine with the vegetables until combined.