For marinade:
2 tablespoons plain non-fat yogurt
1 teaspoon ginger-garlic paste
2 tablespoons tandoori masala paste
2 teaspoons canned pureed tomatoes
1 teaspoon soy sauce
1 teaspoon white vinegar
5 tablespoons lemon juice
1 egg, beaten
2 1/2 tablespoons cayenne pepper, or amount to taste
2 tablespoons ground coriander
2 tablespoons ground cumin
1 teaspoon ground turmeric
1/2 teaspoon salt, or to taste
1/2 teaspoon fresh-ground black pepper

5 (6 ounce) cod fillets, rinsed and dried with paper towels
2 teaspoons butter
2 tablespoons olive oil
2 teaspoons chopped fresh cilantro, for garnish
1 lemon, cut into wedges
1/2 red onion, thinly sliced