Cook the bacon in a skillet on medium-low heat until crisp. Set aside on paper towel-lined plate to drain. When the bacon is cool, crumble and refrigerate.
Mix the mayonnaise, sugar, and vinegar, then add the chopped onion and raisins.
Add the raw broccoli and cauliflower and stir everything well. Pop it in the fridge for 2-5 hours, so the flavors combine and the veggies soften slightly.
Right before serving, add the bacon and cheddar and mix it up.
Be sure to give yourself enough time. Broccoli salad can take a little longer to prepare than other salads, since the raw veggies must chill to soften.