Quarter and core the cabbage. If using regular radicchio, quarter and core that. Very thinly slice the lettuce and cabbage. Halve and very thinly slice the red onion. Place the radicchio, cabbage, and onions in a large bowl. Sprinkle over the orange juice, lemon juice, and balsamic vinegar and toss. Set aside for an hour, tossing every once in a while so that the acids and salt wilt the vegetables a bit. If you are using leafy radicchio, tear them into pieces and set aside.
In a small container, place the olive oil, honey, and thyme. Shake to combine. Pour over the cabbage mixture and toss to combine. If using leafy radicchio, add it now and toss. Add the pepitas. Taste and add salt and pepper as needed.
Notes
Depending on the variety of lettuce you find, you may not want to rush to sliver them up, but look at the leaves and see which you might want to leave whole, or at least in bigger pieces, and which are going to be better slivered up.If you are using regular radicchio and cabbage, it's a good idea to let the lettuces sit in the citrus juices and vinegar for a bit, tossing them every once in a while so that the acids and salt wilt the vegetables a bit. If you are using more delicate lettuces, then dress and toss the cabbage and onions and let that sit for a while. Add the more delicate lettuces just before serving.