Preheat a grill to medium heat.

Season onion rings with salt and pepper and brush with vegetable oil.

In a small saucepan, combine beer and whole-grain mustard.

Bring the beer and mustard mixture to a boil, then reduce heat and simmer.

Add sliced pastrami to the simmering sauce and heat for approximately 3 minutes, until warmed through.

Grill the seasoned onion rings on both sides until softened and slightly charred.

Toast the rye bread on a cooler part of the grill until lightly golden.

Once all ingredients are heated, assemble the sandwich: Spread mustard on one or both slices of bread.

Layer the warmed pastrami and grilled onion rings between the bread slices.

Serve immediately.