Pierce the shell of spaghetti squash with a fork and place in a microwave-safe dish; cook in the microwave on high for 12 minutes. Set aside until cool enough to handle.

Slice squash in half lengthwise; remove seeds. Use a fork to pull the flesh away from the shell and place into a large bowl; fluff with the fork to separate strands as much as possible.

Heat olive oil in a large skillet over medium heat. Cook and stir onion in hot oil until just tender, about 3 minutes. Add garlic; cook and stir for another 3 minutes. Stir zucchini and green bell pepper into mixture; season with Italian herb seasoning and black pepper.

Pour tomatoes into the skillet. Continue cooking just until tomatoes are warmed, 3 to 5 minutes. Add squash strands to the skillet and toss until evenly mixed. Sprinkle with feta cheese and toss again to serve.