Bring a large pot of lightly salted water to a boil. Cook egg noodles in the boiling water, stirring occasionally until cooked through but firm to the bite, about 8 minutes; drain and return noodles to pot.

While the noodles boil, melt butter in a skillet over medium-high heat. Sauté onion and garlic in melted butter until lightly browned and soft, about 8 minutes. Stir baby spinach into the onion mixture; sauté until the spinach is just barely wilted, about 2 minutes.

Stir spinach mixture into the noodles; season with salt and pepper.