Chicken Tostadas with Avocado Sauce

1/2 cup(s) packed fresh cilantro leaves

1 clove(s) garlic , crushed with press

1/2 cup(s) reduced-fat sour cream

1/4 cup(s) fresh lime juice

3 tablespoon(s) water Salt

1 head(s) romaine lettuce , shredded

1/2 small red onion , thinly sliced

1 small orange pepper , seeded and finely chopped

4 corn tortillas

3 cup(s) (from 1/2 rotisserie chicken) coarsely shredded chicken meat

4 plum tomatoes , sliced

1/4 cup(s) crumbled fresh Mexican cheese, such as queso fresco , or feta cheese Lime wedges , for garnish