If you can't find Manchego, try and find another cheese that's piquant but not too strong. Provolone is a great alternative. Although I haven't tried this with mild cheddar, I'm sure you could get away with it if you were in a pinch.
Whilst the linseed is optional, it adds a lovely crunch which works really well with the soft squash. If you have the time, you could follow my recipe for
roasted butternut squash seeds with chilli. These also make a great healthy snack.