Tarragon Tuna Melt

Name: Tarragon Tuna Melt
Ingredients: 1 thinly sliced green onion, including the greens; 3 Tbsp chopped fresh flat-leaf parsley; 2 Tbsp chopped fresh tarragon leaves or 2 teaspoons dried tarragon, crumbled; Zest of one lemon; 2 teaspoons lemon juice; 1 teaspoon Dijon mustard; 1/8 teaspoon freshly ground black pepper; 1/3 cup mayonnaise; 2 6-ounce cans of solid white tuna, well drained and flaked; 8 1/2-inch slices French or Italian loaf bread (or your favorite bread); 1 thinly sliced tomato (8 slices); 4 ounces grated sharp white cheddar cheese (about 1 cup); 2 Tbsp softened butter