Step 1

Heat oil and 2 Tbsp. butter in a large skillet over medium heat until butter is foaming. Add garlic and ¼ tsp. 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.

Step 2

Cook pasta in a large pot of boiling salted water, stirring occasionally, until al dente (about 5 minutes for fresh pasta). Drain.

Step 3

Meanwhile, heat remaining 6 Tbsp. 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.

Step 4

Add pasta to skillet and toss to coat. Add lemon juice, preserved lemon peel, and remaining ¼ tsp. 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.

Step 5

Serve pasta topped with reserved panko.

Step 6

DO AHEAD: Panko can be toasted 1 day ahead. Store airtight at room temperature.