Cold Udon with Grapefruit Ponzu

Name: Cold Udon with Grapefruit Ponzu
Ingredients: 1 grapefruit, halved; 1 lemon, halved; 1 red chile (such as Fresno), seeds removed, finely chopped; 1 2 piece ginger, peeled, finely grated; ⅓ cup unseasoned rice vinegar; 3 Tbsp. soy sauce; 1 tsp. sugar; 2 pints cherry tomatoes; 1 garlic clove, finely grated; 1 Tbsp. extra-virgin olive oil; A pinch of kosher salt; 18 oz. frozen udon noodles; 2 medium Persian cucumbers, peeled, very thinly sliced on a diagonal; Cilantro sprigs, crumbled nori, and toasted sesame seeds (for serving)