Both works! I prefer frozen! You can use frozen spinach if you have it. Remember to thaw spinach completely (either in the fridge, microwave, or on the counter top) and countertop liquids out before using. A few pointers if using fresh spinach – it’s best to use fresh baby spinach. If you can’t find baby spinach use regular spinach and chop it up into smaller pieces. Follow one of the below-mentioned steps : add fresh spinach directly to the sauce in the end and cook it until wilted (but still green) wilt spinach separately by sautéing it in butter in a separate pan until wilted. Once wilted rest the spinach to cool down. Squeeze out liquids and use them in the recipe. blanch spinach in hot/boiling water to cook spinach for 2 minutes. Squeeze out all water then use.