In pan over medium-high heat, sauté onion and yellow bell pepper in olive oil until onion has lightly caramelized. Transfer sautéed vegetables to bowl.
Place mushrooms in same pan and cook until tender. Drain liquid from pan and add mushrooms to bowl.
Toss vegetables with fajita seasoning mix.
To assemble fajitas, warm flour tortillas. Place mixed salad greens over each tortilla, then mound portobello mixture over greens. Top with chopped tomatoes. Spoon nonfat sour cream and salsa over stuffing before rolling each tortilla closed. Cut in half on diagonal.