Peel your onions, carrots, and celery, then rinse them under cold water.

Pat them dry with a clean kitchen towel.

Slice the onions in half from root to tip and dice them into small, roughly 1/4-inch cubes.

Slice carrots into thin rounds about 1/4 inch thick, then stack these rounds and cut them into matchsticks, then chop across the matchsticks to create small cubes.

Slice celery stalks lengthwise into thin strips, then chop across into 1/4-inch pieces.

Place your saucepan over medium heat and add the butter, allowing it to melt fully.

Add diced onions to the hot butter and stir constantly for about 2 minutes.

Stir in the carrots and celery, then add salt and pepper.

Continue cooking, stirring frequently, for another 3 to 4 minutes until the vegetables are tender but still have a slight firmness.

Remove the pan from heat and let the mirepoix cool for a minute or two.