Cook the spaghetti according to the package directions. Drain the pasta, rinse under cold water, and return it to the pot.
Heat the oil and red pepper in a large skillet over medium heat until the pepper is sizzling, 2 to 3 minutes.
Remove from heat and stir in the chopped scallions, tahini, vinegar, soy sauce, 3 tablespoons water, and ½ teaspoon salt.
Add the tahini sauce to the pasta and toss to coat.
Serve topped with the sesame seeds and sliced scallions.