Assemble the salad: In a large bowl, add the spinach and arugula. Add the thinly sliced fennel bulb and the sliced shallots. Finish by adding the orange and grapefruit, both peeled and sliced into rounds.
Make the dressing: In a measuring jug or small bowl, combine 1/4 cup of extra virgin olive oil, the lemon juice, minced garlic, dried oregano, salt, and black pepper. Whisk or stir well to combine.
Serve: Drizzle the prepared dressing over the salad, toss very gently to combine, and serve immediately.