Pre-heat the broiler.
In a large bowl, mash the whole garlic clove with 1/2 teaspoon coarse salt until a paste forms. Add the lemon zest and lemon juice and mix until combined. Add the artichoke hearts, tuna, onion, celery, tomatoes and basil to the bowl and dress with 3 tablespoons of the EVOO (eyeball it). Mash the tuna salad with a fork and season to taste with salt and pepper.
Place the bread slices on a broiler pan and char on both sides under the broiler. Rub one side of each slice with the cut garlic and drizzle with the remaining 1 tablespoon of EVOO. Divide the arugula and the tuna salad among the bread slices and top with the cheese. Return to the broiler and melt the cheese until bubbly, 1-2 minutes.