Sprinkle salmon with lemon juice, salt, and pepper.
Heat 2 tablespoons oil in a skillet over medium heat. Add onion and green bell pepper; cook until softened, about 5 minutes. Add garlic; stir. Add tomatoes; cook until softened, about 5 minutes. Stir in parsley, tarragon, dill, cilantro, and mint. Add cayenne pepper and sugar; season with salt and pepper. Stir in avocado.
Preheat oven to 350 degrees F (175 degrees C).
Pour remaining 2 tablespoons oil into a bowl. Place 1 sheet of phyllo dough on a cutting board, brush with oil and cover with a second sheet. Place salmon on top of pastry, toward the bottom edge. Top with 1 to 2 spoonfuls of the tomato-herb sauce, spreading evenly. Roll the pastry over the salmon; seal edges. Place on a baking sheet.
Bake in the preheated oven until golden, about 45 minutes. Reheat remaining sauce and serve with the salmon.