Dice all of the ingredients into even size pieces.
In a large pot or Dutch oven, heat the oil of choice or rendered bacon fat over MEDIUM heat.
Add the diced trinity (onions, green bell pepper, and celery) to the pot and saute until the vegetables are tender and the onions are translucent - about 8-10 minutes.
Continue on with the beginning of your recipe. The next step is usually the addition of minced garlic.
Notes
NOTES: what you're making will dictate how much of each of the vegetables you will need. When I'm making my Monday Red Beans and Rice, I go with 1 large onion, 1 small green bell pepper, and 3 stalks of celery which is about 1/2 cup worth, and saute it in rendered bacon fat. I like the smoky base that the bacon offers.