Slice the red onion into long thin strips. In a large pot, heat some oil and sauté the onion for 3 minutes until it becomes translucent. Then add the garlic and cook for another minute.
Add the uncoooked spaghetti, the vegetable broth, the coconut milk, and the lemon juice. Stir a couple of times and gently push the spaghetti in the liquid when they're soft enough (this might take a couple of minutes. Try not to break them when they're still too hard). Cook for 10-15 minutes (checking for noodles to soften).
Meanwhile, use a potato or vegetable peeler to make zucchini noodles. Just slice the peeler over the washed zucchini from the top to the bottom. You can keep the skin on. Set aside.
When the spaghetti are al dente, stir in the zucchini noodles and the frozen peas. Cook for 3 more minutes and season with salt, pepper, and red pepper flakes.
