Melt 1 tablespoon unsalted butter in the same skillet over medium heat. Add the onion and garlic, and cook, stirring occasionally until softened, about 3 minutes. Add 3 tablespoons all-purpose flour and immediately stir with a wooden spoon, taking care to scrape the corners, until the flour is completely absorbed, 30 seconds to 1 minute. The mixture will be foamy and there should be no dry pockets of flour left. Pour in 1 1/2 cups low-sodium chicken broth and whisk to incorporate, breaking up any lumps of flour and onion. Pour in 1 1/4 cup whole milk, 1 (1-ounce) packet ranch seasoning, and 1/2 teaspoon kosher salt, and whisk to completely incorporate. Bring to a simmer, stirring constantly, and cook until sauce thickens slightly, about 2 minutes.