Grilling the vegetables--onion, asparagus, sweet pepper, zucchini, and yellow squash--to fill these vegetarian fajitas makes them extra good. Refried beans and cheese add protein.
Heat gas grill to medium-high or prepare charcoal grill with medium-hot coals. In a bowl, stir lime juice, cumin, chili powder, salt, cayenne.
Place onion rings on large plate; brush both sides with some of the lime juice mixture. Microwave on HIGH for 1 minute. Set aside.
Place asparagus in a large resealable plastic bag, peppers in a second bag and squash and zucchini in a third. Divide remaining lime juice mixture among the 3 bags. (Can be done 2 hours ahead.)
Coat all vegetables with nonstick spray and grill, covered. Grill asparagus 10 minutes, turning often, and onion, 8 minutes, turning once. In a grill basket, cook single layer of peppers 10 minutes, turning often; remove. Add squash in a single layer; grill 6 minutes, turning once. Place all vegetables on a platter.
Heat tortillas according to package directions. Spread each with 2 tablespoons hot refried beans, then 3/4 cup mixed grilled vegetables. Top with 1 tablespoon each cheese and guacamole and 2 tablespoons salsa. Fold in half. Repeat with remaining filling and tortillas, and serve.