Beer-Simmered Grilled Sausages

A needle or pin and a cork
3 pounds uncooked sausages, such as sweet or hot Italian sausages,
    bratwurst, chorizo, linguica, or any other sausages you prefer
1 onion, thinly sliced
3 cups beer, as needed
About 1 tablespoon vegetable oil
Mustard, for serving

Prick each sausage a half-dozen times with a needle or pin stuck in a cork. Arrange the onion slices on the bottom of a saut