Gather the ingredients. Preheat oven to 350°F (175°C). Chop 1 yellow onion.
Grate 250 g Cheddar cheese and 150 g Bergeron or Gruyère cheese.
The Bok Choy
Clean and chop to bite size pieces 350 g Baby Bok Choy. Cutting the Bok Choy before blanching ensures it cooks evenly and reduces bitterness.
Bring a pot of salted water to a boil. Add the cut 350 g Baby Bok Choy and blanch for 1-2 minutes until tender and less bitter. Transfer immediately to an ice bath to stop cooking and drain well.
In a oven-safe cooking pot, heat the 1 Tbsp. extra virgin olive oil over medium heat. Add the blanched 350 g Baby Bok Choy with ½ tsp. salt and 1 tsp. sugar and sauté for about 4 minutes or until soft and tender. Remove from heat and set aside. Sautéing after blanching adds flavor and improves texture.
The Pasta
Bring a large pot of salted water to a boil. Cook the 300 g elbow macaroni according to package instructions until al dente, then drain and set aside.
The Cream
While the pasta is getting done, in the same oven-safe cooking pot as the Bok Choy, melt 50 g unsalted butter over medium heat. Add the 1 yellow onion and sauté until translucent about 3-5 minutes. Careful to not burn them.
Stir in the 25 g all-purpose flour and cook for about 1 minutes.
Gradually whisk in the 500 ml half-and-half and 500 ml whole milk, stirring continuously until the mixture thickens.
Stir in the 250 g Cheddar cheese, 150 g Bergeron or Gruyère cheese, 1 tsp. Dijon mustard, 1 tsp. crushed red pepper and ¼ tsp. cayenne pepper until the cheese is melted and the sauce is smooth.
Stir in the blanched and chopped 350 g Baby Bok Choy.Season with ½ tsp. salt and ½ tsp. freshly ground black pepper to taste. Important to taste and adjust.
Add the cooked 300 g elbow macaroni. Sprinkle 2 Tbsp. breadcrumbs on top to taste if desired.
Cover with the lid and bake in the preheated oven at 350°F (175°C) for about 15 minutes, or until the top is golden and bubbly.
The Serving
Let it cool slightly before serving. Enjoy your Mac and Cheese with Bok Choy [Macaroni au fromage et bok choy].