Place a heavy, deep skillet over medium-high heat; add 1 tablespoon olive oil. Once hot, add one-fourth of onions; sauté 1 minute. Add shrimp in a single layer, working in batches if necessary, and cook 3 minutes, or until just opaque, turning once. Transfer to a plate. Return skillet to heat and add remaining 1 tablespoon olive oil. Add remaining onion and garlic and sauté, stirring frequently, about 4 minutes, until translucent. Stir in bell pepper, tomatoes, and chiles (if using); cook 3 more minutes, stirring occasionally. Deglaze pan with reserved shrimp stock, cover, and simmer 10 minutes. Remove and discard chiles.