Put a baking sheet in the middle of the oven; preheat to 450 degrees F. Preheat a grill to high. Mix the melted butter with half of the garlic and 1/4 teaspoon salt in a large bowl; set aside. Spread the fries in a single layer on the hot baking sheet and bake until crisp and golden, about 25 minutes.
Meanwhile, puree the remaining garlic, the parsley, oregano, olive oil, vinegar, 3/4 teaspoon salt, the red pepper flakes and 2 tablespoons water in a blender or food processor to make a sauce; set aside.
Brush the cut sides of the tomatoes with olive oil; season with salt and pepper. Brush the tuna steaks all over with about 2 tablespoons of the herb sauce; season with salt and pepper. Oil the grill grates. Grill the tuna until marked, 2 to 4 minutes per side for medium. Grill the tomatoes cut-side down until marked, about 3 minutes.
Add the fries to the bowl with the garlic butter and toss. Season the tomatoes with salt and pepper. Serve the tuna with the tomatoes and fries; drizzle the remaining herb sauce over the tuna and tomatoes.