While the water for the pasta comes to a boil, we make the sauce. We add the flour to the melted butter, and stir with our flat whisk over medium-low heat for a minute. This butter-flour mixture is known as a roux.
Meanwhile, we heat the milk in the microwave for a minute on full power. We whisk the heated milk into the roux and increase the heat to medium. Using our microplane, the zest of the half of a lemon is added. The juice from the lemon is reserved. We continue to whisk and cook until the mixture comes to a boil. This takes a few minutes.
When the water for the pasta is boiling, we add a pinch of salt and the dried pasta. We cook according to the package directions, checking a few minutes shy of the suggested time. We want the pasta just “al dente”. When it is almost ready, we reserve ½ cup of the pasta cooking liquid, which will be added to the sauce as needed.
When the sauce is boiling, we lower the heat and boil for a few more minutes, stirring well. The peas are placed in a bowl with a teaspoon of water, covered with plastic wrap, and heated in the microwave for a minute. We add them to the sauce, along with some salt and pepper (going easy on the salt until we add the crabmeat and taste). We then gently stir in the crabmeat and lower the heat.
When the sauce is hot, and the pasta cooked, we drain the pasta into a colander and then add it back to the pasta pot. We add the sauce to the pasta along with the some of the pasta cooking liquid, a squeeze of lemon, and the herbs, and serve!