Heat the oil in a large Dutch oven or pot over medium-high heat until shimmering.

Add the onion and sauté until softened and fragrant, about 3 minutes. Add the garlic and orzo, season with salt and pepper, and sauté until fragrant, about 1 minute.

Stir in the broth and milk and bring to a boil. Cover, reduce to a simmer, and cook, stirring occasionally, until the orzo is al dente and most of the liquid is absorbed and has formed a creamy sauce, about 10 minutes. (If the orzo isn’t completely cooked once this has happened, you can add another splash or two of broth to the pot and continue to simmer until it has.)

Stir in the spinach and grated cheeses until the spinach has just wilted and the cheese melts, about 1 minute.

Taste and and add salt & pepper if necessary