Add the cashews, garlic, water, salt and nutritional yeast in a food processor or large blender. Blend until smooth and creamy, scraping down as necessary. Set aside.
Heat a heavy bottomed pot over medium-low heat and add butter. Let it melt then add the 3 tablespoons of gluten-free flour. Stir and let cook for 3-4 minutes until the flour is lightly toasted and a shade darker.
Slowly pour in the cashew cream mixture, whisking as you pour so it doesn't clump up. Add the black pepper and whisk while the cream comes to a simmer. Let simmer until thickened, about 3-5 minutes, then remove from heat. If the mixture remains a little clumpy, just use an immersion blender or an upright blender and blend until smooth.
Store the white garlic sauce in the fridge for up to a week.