Cook pasta according to package directions. Drain well, reserving 1/4 cup (60 mL) cooking liquid.
Meanwhile, heat 2 tbsp (30 mL) oil in large skillet set over medium heat; cook onion and garlic for 3 to 5 minutes or until softened. Sprinkle flour over top; cook, stirring, for 2 to 3 minutes or until smooth.
Slowly whisk in milk; bring to boil, whisking constantly. Reduce heat to medium-low; cook, stirring constantly, for 3 to 5 minutes or until thickened slightly. Stir in mustard, salt, pepper and nutmeg; stir in spinach. Cook for 1 to 2 minutes or until spinach wilts.
Reduce heat to low. Stir in 1 cup (250 mL) cheese for about 30 seconds or until starting to melt. Stir in pasta (mixture should be thick, but if too thick, stir in enough of the reserved cooking liquid for desired consistency). Let cool slightly.
Sprinkle half of the remaining cheese evenly over one side of each tortilla. Divide mac ‘n’ cheese among tortillas. Sprinkle remaining cheese over top; fold tortillas over.
Heat 1 tbsp (15 mL) oil in large skillet set over medium heat; cook quesadillas in batches for 2 to 3 minutes per side or until golden brown and cheese melts, adding remaining oil as needed.