Bring a large pot of water to a boil over high. Meanwhile, heat oil in a medium skillet over medium; add scallions and garlic; cook, stirring often, until garlic is tender and translucent, 3 to 4 minutes. Stir in lemon zest; let sizzle, undisturbed, for a few seconds. Remove from heat.

Add desired amount of salt to boiling water. Stir leeks into boiling water; cook, stirring occasionally, 2 minutes. Add chard and spinach; cook, stirring occasionally, until very tender and deep green, about 1 minute, 30 seconds. Remove from heat. Using a spider, remove leek mixture from water, and hold over pot for a few seconds to drain completely. Transfer drained leek mixture to a blender. Reserve cooking water in pot.

Add scallion mixture, miso, salt, and 1/4 cup cooking water to leek mixture in blender. Process until mixture is very smooth and has a thick heavy cream-like consistency, about 45 seconds. Set aside.

Return cooking water in pot to a boil over high. Add pasta, and cook according to package directions for al dente, adding peas during final 45 seconds cook time. Drain, reserving 1/2 cup cooking water in a small heatproof bowl. Return pasta and peas to dry pot. Add pureed leek mixture to pasta mixture, and toss together, adding reserved cooking water as needed, 1/4 cup at a time, until sauce is loose and creamy. Add lemon juice and additional salt to taste; toss together. Divide pasta evenly among 4 bowls, and garnish with chives and vegan Parmesan, if using.