Gradually whisk in milk to ensure no lumps. Add broth and bring to a boil. Reduce heat to a simmer and cook until thickened. Keep warm.
Meanwhile, in a large skillet, heat olive oil over medium high heat. Add onion and cook until translucent and soft, about 5 minutes. Add spinach, tomatoes and green onions and cook until spinach is just wilted, 2-3 minutes. Add salmon and stir until heated through. Season to taste with salt and pepper.
To serve: Combine pasta with cooked vegetables. Pour sauce over and toss.