Caramelize Shallots and Garlic
Cook shallots and garlic 15-20 minutes until softened and caramelized.
Make Sauce
Cook shrimp until just done. Remove.
Stir red-pepper flakes and anchovies into shallots and garlic for 2 minutes until anchovies dissolve.
Add tomato paste, season with salt and pepper, and stir constantly for 2 minutes until it darkens.
Cook Pasta and Finish
Cook pasta in 2 cups salted boiling water until very al dente.
Transfer sauce mixture to the pasta water.
Cook over medium-high heat for 3-5 minutes, swirling and scraping, until sauce is thick and sticky.
Add spinach until wilted.
Serve topped with parsley, minced garlic, flaky salt, pepper, and optional red-pepper flakes.
