To prepare the batter, heat oil in a kadhai and add the besan. Stir vigorously for a minute, then remove from heat and set aside to cool. After the mixture has cooled, add the remaining batter ingredients.
Coat the fish with the batter and set aside for 20 minutes.
Heat oil in a kadhai and fry the fish until batter is crispy and the fish is cooked. Remove fish with a slotted spoon and drain on kitchen towels.
Sprinkle fillets with garam masala and serve immediately. Sprinkle lemon juice, if using. Garnish with lemon slices and serve.