We love the combination of bitter radicchio and sweet citrus, but you can certainly use your favorite green. Kale and Arugula would definitely work.
Slice the spinach, radicchio, and endives crosswise into thin ribbons and place them in a large bowl. Add the Sumo pieces, lemon juice, remaining 2 tablespoons olive oil, ¼ teaspoon salt, and lots of pepper. Using your hands or two large spoons, toss well to combine. Taste for seasoning and add another pinch of salt or drizzle of olive oil, if needed.
To serve, crumble the goat cheese over the salad and sprinkle the pumpkin seeds on top.