1. Remove seeds from half of serrano pepper; leave seeds in other half of pepper. Place both pepper halves in a mini food processor; pulse 5 times or until minced. Add cilantro and next 9 ingredients (through garlic); process until smooth. Stir in mint.
2. Heat a large nonstick skillet over medium-high heat. Combine garam masala and 1/4 teaspoon salt; sprinkle evenly over fish. Add oil and butter to pan; swirl until butter melts. Add fish to pan; cook 3 minutes on each side or until desired degree of doneness. Serve chutney with fish.
If you're wary of spicy-hot food, remove and discard all the seeds and membranes from the serrano.