Roughly dice bell pepper,onion,and carrot. Halve serranos and de-seed. Place all ingredients except cilantro in roasting pan and drizzle with oil, toss well with salt and pepper and roast at 450° for 30- 40 minutes.
Remove from oven and cool. Working in batches, purée all ingredients in a blender. Transfer to saucepan and reduce by 1/4.