Bring a large pot of salted water to a boil. Add the pasta and cook as the label directs, adding the snap peas during the last 2 minutes. Reserve 1/2 cup cooking water, then drain the pasta and peas.
Meanwhile, heat 3 tablespoons butter in a large skillet over medium-high heat. Add the garlic, 1/2 teaspoon salt and a few grinds of pepper; cook until the garlic is softened, about 1 minute. Add the radishes and bell pepper; cook, stirring, until softened, about 2 minutes. Add half of the reserved pasta cooking water and cook until the vegetables are tender, 2 more minutes. Season with salt and pepper.
Add the pasta and snap peas to the skillet along with the remaining 3 tablespoons butter, the parmesan and all but a few tablespoons of the herbs; toss to coat, adding enough of the reserved pasta cooking water to loosen. Season with salt and pepper.
Combine the ricotta, 1 tablespoon water, the lemon zest and a pinch of salt in a bowl. Top each serving with the lemon ricotta and the remaining herbs.