In a 5- to 6-quart pan over high heat, bring about 3 quarts water to a boil.
Meanwhile, trim stem ends from 1/2 pound yard-long beans; rinse beans.
To boiling water, add beans and 8 ounces fresh udon or fresh Shanghai (about 1/4 in. thick) noodles. Cook just until beans and noodles are tender to bite, 4 to 5 minutes. Drain.
To prepare sesame dressing: In a 6- to 8-inch frying pan over medium heat, combine 2 tablespoons each salad oil and sesame seed. Shake pan often until seed is golden, 3 to 5 minutes. Add 1 tablespoon each minced fresh ginger, minced garlic, and minced fresh red jalapeño or serrano chili (or 1/4 to 1/2 teaspoon hot chili flakes). Stir, remove from heat, and let cool about 2 minutes.
Add 3 tablespoons rice vinegar, 2 tablespoons soy sauce, and 1/2 teaspoon sugar and stir to blend.
Mix sesame dressing with hot beans and noodles.