In a saucepan, combine butter, Dijon mustard, minced onion, poppy seeds, and lemon juice.

Simmer the mixture until the onions are softened.

Slice the French or Italian bread into 1-inch thick slices.

Spoon the butter mixture generously onto each bread slice.

Add cubed Monterey Jack cheese on top of each slice.

Wrap each slice in foil, leaving the top exposed.

If preparing ahead, chill the wrapped bread slices.

Preheat oven to 350 degrees Fahrenheit.

Bake for 20 minutes, or until cheese is melted and bubbly and the bread is heated through.