Bring a large pot of water to a boil. When the water has come to a boil, heavily salt the water.

Cook the spaghetti according to the package directions till al dente.

While the pasta is cooking heat a large coverable skillet over medium heat.

When the skillet is hot, add the olive oil and the butter. Let the butter melt.

Add the chopped red onion and the chili flakes. Cook until the onion has sweated and softened.

Add the asparagus and a couple of spoonfuls of the water from the cooking pasta.

Cover and let cook on simmer for a few minutes while the pasta finishes cooking.

When the spaghetti has finished cooking to the al dente stage, remove the pasta by lifting with tongs into the skillet or a basket strainer. You want some of the pasta cooking water to go along with the cooked spaghetti to make the sauce. You can strain it in a colander. But be sure to keep a little of the cooking water for finishing the dish.

Turn off the heat and toss in the parmesan cheese and the grated lemon zest. Continue to toss until the cheese has blended with the pasta water and made a creamy but thin sauce.

Squeeze in the juice from the lemon (be careful not to get any seeds, you can use a towel or strainer to keep the seeds out if needed.) Toss the pasta again briefly.

Serve immediately.