Tunacado Sandwich

Name: Tunacado Sandwich
Ingredients: 10 ounces tuna (drained (preferably a good chunky tuna in olive oil), (usually 2 cans that are 5 oz each)); 1 Tablespoon olive oil ( or butter); 1 medium onion (, or 2 shallots chopped); 1/2 teaspoon salt; 3 Tablespoons mayonnaise (, or to personal taste); 1 stalk celery (, diced); 2 Tablespoons diced pickles or relish ((sweet or dill depending on your preference)); 2 Tablespoons minced fresh dill (, or 1 teaspoon dried dill); 2 Tablespoons minced parsley; 1 Tablespoon mustard (, or more to taste); fresh cracked black pepper (, to taste); zest from 1 medium lemon; 2 teaspoons fresh lemon juice; 1 teaspoon soy sauce; 1/2 teaspoon hot sauce ( or Sriracha sauce - optional); 1 jalapeno (, sliced - optional); 8 slices sourdough bread (or bread of choice); 4 Tablespoons olive oil ( or butter); 1/2 cup basil pesto; 1 large avocado (, sliced); 1 large tomato (, sliced ); 4 slices melting cheese like cheddar ((optional))