Heat a large heavy skillet or saucepan over medium heat. Add olive oil and butter. When butter melts and foam subsides, add the chopped onion and sauté 4–5 minutes until translucent and soft, stirring occasionally.

Cook garlic

Add minced garlic to the pan and cook 30–45 seconds until fragrant, stirring constantly to avoid browning.

Wilt spinach

Add the fresh baby spinach in batches, tossing with tongs so it wilts down evenly. Continue until all spinach is added and mostly wilted, about 2–3 minutes. It will release liquid.

Liquid & Thickening

Make a slurry

Sprinkle the tablespoon of flour over the wilted spinach and stir for 1 minute to cook the raw flour taste.

Add liquids

Pour in the cup of broth while stirring to combine, then add the half-and-half (or whole milk). Bring to a gentle simmer and cook 3–4 minutes, stirring occasionally, until the mixture slightly thickens.

Seasoning & Add-ins

Season and finish

Stir in the salt, black pepper, nutmeg, red pepper flakes (if using), and lemon juice. Continue simmering 1–2 minutes to meld flavors.

Blend (optional)

For a smoother, creamier spinach dish (soup or casserole base), transfer the mixture to an immersion blender and pulse until you reach the desired texture. Alternatively, leave it chunky for a salad/casserole topping.

Add cheese and serve

Return mixture to the pan if blended. Stir in the grated Parmesan until melted and fully incorporated. Taste and adjust seasoning with additional salt, pepper, or lemon as desired. Serve warm as a side, as a creamy soup bowl (add extra broth to thin), or use as a casserole layer.