Toast bread: Toast your bread and set aside.
Prepare tuna mousse: In a small bowl, add drained tuna, red onion, capers, dill, mustard, greek yogurt, lemon juice, salt, and pepper. Mix very well with a fork until you have an almost smooth tuna salad. If desired, you could prepare this in a food processor instead (but I prefer a bit of texture remaining).
Assemble sandwich: Spread pesto on one slice of bread. On the other slice, spread mashed avocado. Add tomato slices, and season with salt and pepper. Top with tuna, close your sandwich, and slice as desired.
Nutrition facts calculated with 1 Tbsp of pesto.
Calories: 490kcal, Carbohydrates: 36g, Protein: 47g, Fat: 17g, Saturated Fat: 3g, Polyunsaturated Fat: 2g, Monounsaturated Fat: 6g, Cholesterol: 65mg, Sodium: 954mg, Potassium: 888mg, Fiber: 8g, Sugar: 7g, Vitamin A: 744IU, Vitamin C: 16mg, Calcium: 226mg, Iron: 5mg