Heat the oven to 400ºF. Whisk the eggs vigorously with the chives, lemon zest and salt and pepper to taste. Heat an ovenproof 10-inch enameled cast-iron or nonstick skillet over medium-high heat. Pour the eggs into the pan and stir in the smoked trout and avocado. Let the eggs set for several seconds, and then, with a heatproof spatula, gently stir the eggs, starting from the center. This will make the frittata puff up more in the oven. Lift the edges so that the eggs flow to bottom. Transfer the pan to the heated oven. Cook for about 10 minutes, until puffed and golden.