Bring a pot of well salted water to boil and cook the pasta according to package instructions. When the pasta is ready, drain it and rinse it under cold, running water.
While the pasta cooks, bring another pot of water to boil and fit it with a steamer attachment. Steam the broccoli, peas, and cut green beans till the broccoli is tender but still has some crunch (about 5-7 minutes).
Whisk together the dressing ingredients. The mixture should be slightly thicker than a regular salad dressing. If it's very thick, add water by the tablespoon until it's easy to stir.
When the pasta is ready, transfer it to a large mixing bowl. Add the peas, green beans, broccoli, pepper, chopped green onion tops if using, and any other mix-ins you like. Add the tahini dressing and mix everything really well. Then taste the pasta and add salt, black pepper, and extra lime juice or vinegar as desired. Serve.