- Preheat oven to 425°F (218C). Line a rimmed baking sheet with aluminum foil.
- In a medium bowl, toss the squash with 1 tablespoon olive oil, a pinch salt and 1⁄4 teaspoon black pepper.
- Spread on the prepared baking sheet and roast, stirring once for 15 minutes.
- Remove the pan from the oven and move the squash to one side. Place the salmon fillets on the foil and sprinkle each with salt and pepper.
- Return to the oven and bake 5 to 10 minutes, turning halfway through cooking time. The salmon is finished when it flakes easily with a fork.
- Meanwhile, in a small bowl, whisk together the lemon juice, olive oil, garlic, tarragon and Dijon mustard.
- In a large bowl, add the spinach and half of the vinaigrette and toss to combine.
- Divide the spinach between four serving plates, top with 1⁄4 of the butternut squash and a salmon fillet and drizzle with some of the vinaigrette.