Heat oil and 2 tablespoons butter in a large skillet over medium heat until butter is foaming. Add garlic and 1/4 teaspoon red pepper flakes; cook, stirring often, until fragrant, about 1 minute. Add panko and cook, stirring often, until panko is golden brown, about 2 minutes. Mix in lemon zest and transfer panko to a paper towel-lined plate; season with salt and pepper. Let cool; set aside. Wipe out skillet.
Cook pasta in a large pot of boiling salted water, stirring occasionally, until al dente (about 5 minutes for fresh pasta). Drain.
Meanwhile, heat remaining 6 tablespoons butter in same skillet over medium heat. Cook, swirling skillet occasionally, until butter is brown, about 3 minutes. Add 1 bunch spinach; cook, tossing, until wilted, about 1 minute.
Add pasta to skillet and toss to coat. Add lemon juice, preserved lemon peel, and remaining 1/4 teaspoon red pepper flakes and toss to combine; season with salt, pepper, and more lemon juice and preserved lemon peel, if desired. Add remaining spinach and toss until slightly wilted, about 1 minute.
Serve pasta topped with reserved panko.
DO AHEAD: Panko can be toasted 1 day ahead. Store airtight at room temperature.
