Preheat the oven to 425°. Boil the conchiglie pasta until al dente in a large pot of boiling salted water, which takes about 8-9 minutes while gently stirring.
Add the olive oil to a large rondeau pot over medium heat and cook the onions until lightly browned, which takes about 10 minutes. Next add in the garlic and cook for 1 minute.
Mix in the baby spinach just until wilted and then transfer the mixture to a large plate and slightly spread everything out and chill in the refrigerator for about 30 minutes.