Combine the arugula, tomatoes, onion, and fennel in a bowl. Using two wooden spoons, toss to combine. Drizzle the olive oil evenly over the salad and toss well to coat the leaves with the oil. Drizzle the lemon juice evenly over the salad and toss well to combine. Sprinkle with salt and pepper, then toss well to combine. Taste for seasonings.