Step 1

Heat broiler. Squeeze grapefruit and lemon halves into a fine-mesh sieve set over a large bowl; discard pulp (you should have a little more than ½ cup juice). Mix in chile, ginger, vinegar, soy sauce, and sugar. Cover ponzu and chill.

Step 2

Toss tomatoes, garlic, oil, and salt on a rimmed baking sheet. Broil until tomatoes are soft and lightly charred, 10–12 minutes. Let cool.

Step 3

Cook noodles according to package directions; drain and rinse under cold water to stop the cooking. Transfer noodles to bowl with ponzu and toss to coat. Divide noodles and ponzu among bowls and top with charred tomatoes, cucumbers, cilantro, nori, and sesame seeds.