Heat ¼ cup oil in a large skillet over medium heat. Add walnuts and fry them till deeply golden brown, smelling almost like popcorn, 2–3 minutes. Using a slotted spoon if you have it, remove nuts (leaving oil behind) and transfer to a small bowl; season with salt and set aside.
Thinly slice about ⅓ of the scallions and set aside. Coarsely chop the remaining scallions (light and white green parts-- the whole thing!) and add to the skillet with oil. Season with salt and pepper and cook over medium-high heat until they're lightly browned at the edges and completely tender, bordering on "melty," 8–10 minutes. Add crushed red pepper flakes if using, then add half the zucchini.
Season zucchini with salt and pepper and let it cook down until tender and translucent, 10–15 minutes. This time will vary depending on the zucchini-- larger squash has more water and will take longer to get to that melty translucent stage, but just stir or shake your skillet occasionally, taste one every now and then, and be patient. Once it's there, add the remaining zucchini (you should have more space in your skillet now) and season again with salt and pepper. Add capers and cook zucchini until it's wilted and tender, but still has a little bite, another 5–8 minutes or so (you want some very soft, tender zucchini and some less so).
Meanwhile, cook pasta in a large pot of salted water until just past al dente (you aren't cooking it again, so it should be basically fully cooked without feeling mushy). Drain and rinse under cold water; set aside.
Transfer the zucchini mixture to a large mixing bowl and add garlic, the juice of 1 lemon, and remaining thinly sliced raw scallion; season everything with salt and pepper. Add pasta and gently toss to coat (I don't even like using a spoon here, just some casual bowl tossing). Season again with more crushed red pepper flakes and lemon juice if you think it needs it.
Add half the parmesan, walnuts and herbs and toss to coat. Transfer to a serving bowl (you can also serve out of the bowl you just made it in) and top with remaining parmesan, nuts and herbs.