Cut the radicchio into quarters, remove the stem, then cut it into thirds. In a large bowl or a salad spinner, cover the radicchio with ice water. Use your hands to massage the leaves, then let it soak for 15 minutes. Drain and dry well.
While you wait for the radicchio to soak, make the easy homemade ranch dressing.
Next, prepare the salad toppings. Cut the bacon slices into 1/2 inch pieces and cook in a medium skillet over medium heat. Stir occasionally until most of the fat has rendered and the pieces start to brown and crisp (10-12 minutes). Remove the bacon pieces and set them on a paper towel-lined plate.
Slice the sourdough bread into crouton sized pieces. Toss them in olive oil and spread on baking sheet. Bake for 4-5 minutes, watching them to make sure they don’t burn.
Slice the red onion into thin slices and set aside. Crumble the blue cheese.
Assemble the salad in a large serving dish. Add the chopped radicchio, the bacon pieces, croutons, and cheese. Drizzle the ranch over top and toss everything together.
Serve as a main dish for 2 people or a side for 4.