The Jambon-Fromage-Beurre is the maximal version of the everyday French ham sandwich: all three elements present at once, ham, cheese, and butter, with nothing left implicit. The ham-and-cheese baguette often treats butter as optional, since the cheese already supplies fat. This sandwich puts it back deliberately and in quantity. The defining choice is that refusal to economize, the decision that the butter earns its place even when a cheese is already there.
The build is a baguette, a thick spread of barely-salted butter, slices of jambon de Paris, and slices of a firm cheese such as Emmental or Gruyère. Each part does something the others do not. The butter cools and rounds the bread's salt and adds a clean dairy note the aged cheese cannot. The cheese brings savory depth and chew. The boiled ham keeps the center gentle and slightly sweet so the richer elements have something restrained to play against. The risk in a three-fat construction is heaviness, and the sandwich answers it by keeping the ham mild and the cheese in proportion rather than slabbed.
The craft is in proportion and order. The butter goes directly on the crumb so it meets the bread rather than the cheese; the ham sits in the middle as the quiet layer; the cheese is sliced thin enough to fold rather than wedge. Get the ratio wrong and the butter and cheese collapse into one undifferentiated richness; get it right and you can still taste three distinct things in a single bite. The baguette still has to be fresh and the crust still has to crack, and the sandwich is still best eaten soon after assembly rather than held.
Variations are adjustments to which of the three leads. A more aged Comté makes the cheese dominant; a thicker butter layer with a milder cheese makes the dairy the throughline; a dry-cured ham shifts the weight to the meat. Each is a rebalancing of the same three parts. The sandwich they all descend from is the Jambon-Beurre, here carrying both cheese and a full measure of butter at once.