You can make this salad up to one day ahead, just wait to add the radicchio and cheese until ready to serve.
1/2 small red onion, sliced thin
2 Tbsp. lemon juice
1 Tbsp. dried oregano
Salt and pepper
2 Tbsp. extra-virgin olive oil
2 Tbsp. red wine vinegar
1/2 head radicchio, cored and sliced into 1"-long pieces
1 (15-oz.) can chickpeas, drained, rinsed, and patted dry
1 (15-oz.) can red kidney beans, drained, rinsed, and patted dry
4 oz. green beans, cut into 1" pieces (about 1 cup)
1/2 cup pitted Kalamata olives, halved lengthwise
1/2 cup marinated artichoke heart quarters, halved lengthwise
1/2 cup pepperoncini peppers, sliced into 1/4"-thick rings
4 oz. provolone cheese, cut into 1/4" cubes