Step 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.
Step 2
Set an oven rack about 6 inches from broiler's heat source and preheat oven's broiler.
Step 3
Line a baking sheet with aluminum foil.
Step 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.
Step 5
Cook under the preheated broiler until the skin of the peppers has blackened and blistered about 5 minutes.
Step 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.
Step 7
Turn oven's temperature to 500 degrees F (260 degrees C).
Step 8
Remove portobello mushroom caps from bag shake off excess vinegar and arrange onto a baking sheet.
Step 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.
Step 9
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.