Heat a large skillet over medium heat. Add 2 tablespoons unsalted butter and let it melt, swirling the pan, about 30 seconds.
Add 3 cloves minced garlic and sauté, stirring constantly, until fragrant and just beginning to soften, about 30–60 seconds. Do not let the garlic brown.
Add 12 oz (350 g) baby spinach (in batches if the skillet is small). Stir and toss the spinach until it is wilted and reduced in volume, about 2–4 minutes.
Pour in 2/3 cup heavy whipping cream and 1/3 cup water. Add 1/4 teaspoon salt and 3 dashes ground black pepper, then stir to combine so the spinach is evenly coated.
Reduce the heat to low and simmer gently, stirring occasionally, for about 5 minutes, until the spinach is very soft and the sauce has slightly thickened but the greens remain a vibrant green.
Remove from the heat and serve immediately.