Preheat oven to 400 degrees. Place the water in a large pot and bring to the boil. Add the salt, return to the boil, and add the pasta. When the water returns to the boil, begin to time the pasta carefully, and after 8 minutes of cooking time add the leeks. Cook the pasta and leeks together for another 2 minutes after the water has returned to the boil.
While waiting for the pasta to cook, put the butter into a saucepan and begin to melt to a golden brown. When the butter is a light nut brown add the sage and poppy seeds and set aside.
The pasta leek combination should cook for a total time of about 10 to 11 minutes (check the recommenced cooking time on the pasta box). Add the spinach in the final 2 minutes of cooking time. Cook together for 2 to 3 minutes, drain and toss with the sage butter, add the salt and pepper and place into a buttered gratin dish.
Add both cheeses and stir to combine. Place the gratin dish into the oven and bake for 10 to 12 minutes until the cheese has melted and the top edges begin to brown.
Serve immediately.