Measure out your fat and flour. For a basic roux, the ratio is typically equal parts by volume, so 1/4 cup of fat to 1/4 cup of flour is a great starting point for about 2 cups of liquid.

Place a heavy-bottomed saucepan over medium-low heat. Add the fat and allow it to melt completely. If using butter, let it melt gently without browning.

Once the fat is melted and shimmering (but not smoking), gradually whisk in the flour. Continue whisking constantly to combine the flour and fat, ensuring there are no lumps. This is your white roux.

For a blonde roux, continue cooking and whisking for about 3-5 minutes. The mixture will turn a pale, straw-like color and emit a slightly nutty aroma. Be careful not to burn it.

To achieve a brown roux, continue cooking and whisking for another 5-10 minutes (totaling 8-15 minutes from the start). The roux will deepen to a peanut butter or light brown color and have a more pronounced nutty scent. This adds more flavor but has less thickening power.

For a dark brown or 'gumbo' roux, continue cooking and whisking for an additional 5-15 minutes (totaling 15-30 minutes or more). The color will be a deep, rich brown, similar to dark chocolate. This roux has the most flavor but the least thickening ability. Constant stirring is crucial to prevent burning.

Once your desired color is reached, you can immediately add your liquid (stock, milk, etc.) to create your sauce or gravy. Whisk vigorously as you add the liquid to incorporate the roux smoothly and prevent lumps.