Use a food processor or a sharp knife to shred the cabbage and carrots. Set them aside in a large mixing bowl.
In a large skillet, cook the bacon over medium heat until it is crisp. Remove the bacon from the skillet and place it on a paper towel to let it drain. Set aside.
In the same skillet with the bacon grease, add the chopped medium onion and cook until softened and slightly translucent over medium heat (about 5 minutes).
Stir the shredded cabbage and carrots into the skillet with the onions. Cook for about 5 minutes, occasionally stirring to ensure even cooking. Season with salt and black pepper during this step.
In a small saucepan, combine the apple cider vinegar, sugar and celery seed. Heat the mixture over low heat, stirring until the sugar is fully dissolved.
Slowly whisk in the vegetable oil and yellow mustard into the vinegar mixture. Continue heating the sauce on low until it thickens slightly.
Once the dressing is heated and thickened, carefully pour it over the cabbage mixture in the skillet. Toss gently to coat the vegetables evenly with the hot dressing.
Top the hot slaw with the cooked bacon and green onions, if desired. Serve immediately while the slaw is still warm and enjoy your Old Fashioned Hot Slaw!