Bring a large pot of salted water to a boil. Add the cauliflower and cook until just tender at the edges, 3 to 4 minutes. Drain well.

Transfer the cauliflower to a parchment-lined sheet pan. Toss with 2 tablespoons of olive oil and a pinch of salt. Roast until golden and lightly charred, about 30 minutes.

Meanwhile, toast the pumpkin seeds in a dry skillet over medium heat until fragrant, 2 to 3 minutes. Let cool.

In a large bowl, whisk together the remaining 2 tablespoons olive oil, lemon juice, sherry vinegar, salt, and pepper.

Add the arugula, red onion, and warm cauliflower. Toss gently to coat. Transfer to a serving platter, scatter the pumpkin seeds over the top, and serve.