Cook the vegan chicken and the pasta, separately, according to package directions, and place the spinach into the colander. When you strain the pasta, use a large spoon to toss it with the spinach in the colander to cook it through. When the vegan chicken is cooked, set it aside.
Heat the olive oil in a saucepan on medium-high heat. Add the onion and garlic and cook, stirring, until the onion softens, about 5 minutes.
Reduce the heat to medium-low, add the flour and cook, stirring constantly, until you've got a thick paste. This takes about 3 minutes.
Slowly add the vegan milk, whisking out any lumps as you go, then add the basil. Bump the heat up to medium, and simmer, stirring frequently, for 10-15 minutes. It's okay if the milk boils as long as you are stirring it often. When the milk has thickened enough to pass the "spoon test" (See note.), you're ready to move on.
Remove the pan from the heat, and while the milk mixture is still hot, add the shredded vegan cheese, stirring to melt. Add the cooked pasta-spinach mixture and the cooked vegan chicken, stirring to get a nice, even mix. Taste, and season with salt and pepper, then use your spoon to smooth out the top of the casserole.
Place the pan into the preheated oven and bake for 20-30 minutes, until the top browns slightly and the edges look nice and bubbly.
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Notes
About the flour: If you don't have or can't use all purpose flour, you can substitute the same amount of chickpea flour, whole wheat flour, or white wheat flour.
What is the spoon test? To see if your cream sauce is thick enough, drag a wooden spoon across the bottom of the pan. If it takes a second or two for the sauce to fill in the space the spoon made, your sauce has passed the spoon test!
About the vegan chicken strips: You can use any brand of vegan chicken strips that you like. Chicken style seitan will also work. Since cooking methods can vary by brand, I'm recommending that you cook according to the package directions.