How to Make It
Preheat the oven to 225°. Arrange the plum tomatoes and half of the cherry tomatoes cut side up on a rimmed baking sheet. Drizzle the tomatoes with olive oil and season with salt and pepper. Scatter the garlic slices on top. Turn the plum tomatoes cut side down. Bake the tomatoes for about 2 1/2 hours, or until they are shriveled, but still moist. Transfer the tomatoes to a bowl. Pull off and discard the plum tomato skins.
Meanwhile, puree the remaining cherry tomatoes in a food processor. Working over a medium bowl, pass the puree through a fine mesh strainer, pressing on the solids. Stir the red wine vinegar into the tomato jus and season with salt.
Light a grill or heat a grill pan. Lightly rub the mahimahi fillets with olive oil and season the fillets with salt and black pepper. Grill the mahimahi over high heat, turning once, until lightly charred and cooked through, about 8 minutes.
Pour the tomato jus onto 4 plates and set the mahimahi fillets on top; spoon the roasted tomatoes alongside. Garnish with the basil leaves and serve.