Boil pasta in a large pot of well-salted water for 7 minutes, or until slightly firmer than al dente. Drain spaghetti, reserving 1/2 cup cooking water. Return spaghetti to the pot.
Whisk together tahini, soy sauce, brown sugar, garlic, lemon juice and chili flakes in a large measuring cup. Thin with reserved hot pasta water until it reaches a pourable consistency.
Squeeze excess moisture from spinach and add to the pot along with the sauce, and gently toss to combine. Cook over low heat until warmed through. Taste and adjust seasoning as desired.
Serve topped with toasted sesame seeds and crushed chili flakes, if desired.