Heat a large skillet over medium-high heat and add the butter. Once melted add in the frozen chopped spinach (no need to defrost prior). Stir, until heated through and no longer frozen.: The moment the butter melts you will notice a soft, slightly nutty aroma that signals the beginning of the build. Listen for a gentle shimmer across the surface rather than aggressive bubbling, which tells you the pan is at a good medium high. If the butter starts to brown quickly, reduce the heat right away, because browned butter will change the finished flavor and can make the sauce taste more toasted than creamy. A common misstep is cranking the heat to rush things, which leads to uneven cooking of the spinach once added.
Add in the minced garlic and cook until fragrant. Stir in the all purpose flour and cook an additional 2 to 3 minutes, until no flour is visible. Gradually stir in 1 1/2 cups heavy cream and reduce heat to medium.: As you pour in the frozen spinach you will hear a soft sizzle and see steam rise, and the block will quickly lose its icy sheen. Stir continuously so the leaves separate and heat evenly, watching for a uniform deep green color that shows the spinach is warmed through. If excess liquid collects, tilt the pan and spoon off a little to prevent a watery sauce, because too much released water will thin the roux later. Avoid pressing the spinach flat right away, which can squeeze out juices prematurely.
Stir in the parmesan cheese, onion powder, salt, pepper and crushed red pepper. If needed, add in up to 1/2 cup more heavy cream until desired creaminess is reached. Serve right away and enjoy!: When the spinach is pliable and steaming, its texture shifts to tender and the aroma becomes vegetal and sweet. Continue stirring so pockets of ice do not remain, and aim for a uniformly heated mass. If you stop stirring, some clumps may remain colder and create uneven sauce consistency later. The mistake many make here is assuming visual thaw equals fully warmed, so touch a small piece to confirm warmth.
Add in the minced garlic and cook until fragrant: Once the pan is back to a gentle sizzle, add the minced garlic and breathe in the immediate fragrant pop as it releases its oils. That scent usually lasts only a minute, so stir briskly and watch for a translucent, toasty note rather than dark brown bits. Burnt garlic becomes bitter and will dominate the sauce, so as soon as you smell it, move to the next step. If you accidentally scorch the garlic , remove the pan from heat briefly and scrape out the burned pieces.
Stir in the all purpose flour and cook an additional 2 to 3 minutes, until no flour is visible: The flour will create a bonding matrix for the cream, and as you stir you should see the raw powder disappear and the mixture bubble gently. The aroma will shift to a lightly toasted, bready note which means the flour is cooked and will not carry a raw taste. Stir continuously to prevent clumping, and if small lumps form, press them against the pan with your spoon to smooth them out. Rushing this step can leave behind a pasty aftertaste in the final sauce.
Gradually stir in 1 1/2 cups heavy cream and reduce heat to medium: Pour the heavy cream slowly while stirring to integrate the liquid without shocking the roux, and you will notice the sauce begin to thicken and gloss over the spinach . Reducing to medium heat prevents rapid boiling that can separate the cream, and the surface should show slow, even bubbling. If the sauce seems too thin, simmer gently and whisk until it reaches the silkiness you want. Adding all the cream at once can create pockets of unincorporated liquid that make emulsification harder.
Stir in the parmesan cheese, onion powder, salt, pepper and crushed red pepper: As the parmesan cheese melts into the warm sauce it will create a slightly grainy, savory layer that rounds the taste. Mix in the onion powder , salt , black pepper , and crushed red pepper for balanced seasoning. Taste and adjust carefully because the parmesan cheese brings saltiness that accumulates. A frequent error is over seasoning before the cheese melts, so always re taste after the cheese is fully incorporated.
If needed, add in up to 1/2 cup more heavy cream until desired creaminess is reached: Texture is personal, so add extra heavy cream incrementally and stir to judge thickness. Each addition will soften the sauce and make it silkier, and you should aim for a coating consistency where the sauce clings but does not puddle. If you overshoot and it becomes too thin, simmer gently to reduce and concentrate. A common mistake is adding cold cream in large quantities, which can temporarily cool the pan and slow melting, so warm the cream slightly if possible.
Serve right away and enjoy: The final dish should glisten and give off a warm, savory aroma with hints of garlic and parmesan cheese . Serve immediately so the texture remains creamy and the flavors are most vivid. If held too long on gentle heat, the sauce may thicken or separate slightly, so if you must hold it, give a quick stir with a splash of cream before plating. Reheating briskly over low heat with a touch more liquid helps restore silkiness.