Combine the onion, lemon juice, capers, oil, and 1/4 teaspoon salt and pepper in a bowl. Let stand until the onions soften slightly, about 10 minutes.
In another bowl, mash the lemon zest and avocado with 1/8 teaspoon salt.
Spread half of the avocado mixture on one slice of toast and top with half the onions. If desired, add either the smoked trout or the soft-cooked egg. Repeat with remaining ingredients, and serve immediately.