Pasta with Fennel, Kale, & Lemon

extra virgin olive oil

large red onion, thinly sliced

fennel bulbs, (tough outer layer removed and fronds reserved) thinly sliced

large garlic cloves, finely slivered

large pinches red pepper flakes

large bunch of lacinato kale, tough stems removed and roughly chopped

juice of one lemon

spinach fettuccine

salt

pepper

Parmigiano-Reggiano cheese, grated

fennel fronds

chili pepper oil (optional)