Prep time is about 45 minutes, and the cost is way below what you'd pay for a sandwich like this in a nice café. A block of gruyère is more expensive than swiss or cheddar, but so worth it, and you'll be able to get six to eight sandwiches out of a 1"x4" chunk, or use the remaining cheese for anything else you like.
-- 1 medium sweet yellow onion, very thinly sliced
-- several slices (about as much as you would use for a typical grilled cheese) of delicious Gruyère cheese
-- 2 ciabatta rolls from your grocery's bakery
-- 2 thin slices of ham or turkey (optional), according to your preference
-- about a teaspoon of olive oil
-- honey mustard or the dressing of your choice
-- a small cube (say about 1/2 teaspoon or so) of salted real butter
-- panini grill or similar appliance, preferably with flat griddle plates rather than ribbed grill ones
6. Spread a small amount of honey mustard or your preferred dressing on each side of both ciabatta rolls.
9. Let paninis heat for about 10 minutes, until you can tell that the cheese is melty and everything inside is toasty warm.