Lightly wipe the caps clean with a paper towel. Cut into 1/2-inch slices, place in a large zip-top bag and add 3/4 cup of balsamic dressing. Squeeze air from bag and seal. Lightly massage the mushrooms to ensure they are all evenly coated. Marinate in refrigerator for 1 to 2 hours.
Heat olive oil in a large saute pan over medium high heat. Add the marinated mushrooms and saute for about 6 to 8 minutes until tender.
Meanwhile in a small bowl, stir together ricotta, 2 tablespoons balsamic dressing, and Italian seasoning; season with salt and pepper and set aside.
Spread crostini with the ricotta mixture. Top each crostini with a portobello slice and cross with a red pepper strip. Garnish with oregano leaf.