Bring a medium pot of salted water to a boil.
Add the spinach to the blender, pushing it down toward the bottom to fit it all. Carefully ladle out 1/4 cup of the boiling water and add it to the spinach. Blend until smooth, about 10 seconds. Do not over blend or it will muddy the vibrant color. Set aside to cool.
Add the pasta to the boiling water and cook until al dente according to the package directions. Drain and set aside.
Meanwhile, toss together the pepper jack, Sage Derby cheese and cornstarch in a bowl until well combined.
Set a medium saucepan over low heat. Add the evaporated milk and bring to a simmer. Whisk in the cheese mixture and cook, stirring constantly, until the cheese is melted and the sauce is silky and smooth, about 5 minutes. Off the heat, whisk in the spinach puree, then toss with the pasta.