Preheat the oven to 375°F. Place the fennel on a rimmed baking sheet, preferably lined with foil or parchment paper. Drizzle over 2 tablespoons of the olive oil, toss, and spread out in a single layer. Season with salt and pepper, sprinkle with the Parmesan, and roast for 40 to 45 minutes until browned and tender. Let cool on the baking sheet to room temperature.
Meanwhile, make the salad. In a large bowl, combine the remaining 4 tablespoons (1/4 cup) olive oil, the lemon juice, vinegar, and salt and pepper to taste. Stir then add the spinach, arugula, and red onion and toss to coat with the dressing. Add the cooled fennel and toss again. Sprinkle with the pepita seeds and serve.
Notes
Summer salads kind of make themselves, but cold weather salads take thought, and can actually be more interesting. You can also take this as an invitation to consider adding leftover roasted vegetables to other leafy green salads. I love the textural contrast, and cooked vegetables make green salads feel instantly more robust, even main course-worthy.I might also serve this salad with some slices of leftover cold chicken on top, for a luxurious lunch.