Cover the bottom of a small skillet with the cumin; cook and stir over low heat for 2 minutes or until lightly toasted. Remove from heat. For chimichurri, combine the cumin, cilantro, parsley, vinegar, red pepper, and sea salt or coarse salt in a blender container. Cover and blend on lowest speed, slowly adding the oil.
Combine ground beef, the 1/4 teaspoon salt, and the ground red pepper in a large mixing bowl. Shape into 12 patties 3-1/2 inches in diameter. Spoon about 1 tablespoon of the chimichurri mixture in the center of each of 6 of the patties. Place remaining patties on top and seal edges.
Place burger patties on the lightly oiled rack of the grill directly over medium heat. Grill for 18 to 22 minutes or until an instant-read thermometer inserted in centers registers 160 degree F, turning once halfway through grilling. Brush rolls with some of the chimichurri sauce. Top with burgers, remaining sauce, and tomatoes and onions. Serve with Grilled Plantains, if desired. Makes 6 burgers.
To make the Grilled Plantains, bias-slice peeled plantains into 1/2-inch-thick slices. Brush with peanut oil. Grill directly over medium heat for 8 minutes or until centers are just soft, turning once. Drain on clean paper towels. Serve warm.