Gazpacho with Serrano Chiles

4 large red bell peppers, about 1 1/2 lb. total  

1 1/4 lb. tomatoes 

1/2 English cucumber, peeled and cut into large chunks 

1/2 yellow bell pepper, seeded and cut into 1/4-inch dice 

2 serrano chiles, seeded and minced 

1 small red onion, finely chopped 

1 garlic clove, minced 

2 Tbs. extra-virgin olive oil, plus more for drizzling 

1 Tbs. sherry vinegar 

1 tsp. salt 

1 avocado, halved, pitted, peeled and cut into 1/4-inch cubes 

1 Tbs. snipped fresh chives 

1/2 tsp. dried oregano