Wash the tomatoes, grate them dry and cut them into slices. Peel onions and cut into thin rings. Cut bread into 4 pieces. Cut each piece in half horizontally once. Brush the lower halves with butter.
Fold the ham slices once so that they are as wide as the baguette. Halve the cheese slices. Cover the baguette with ham and cheese. Spread tomatoes and onions on top. Spread mustard on the top of the baguette halves and use them as a lid.
Line the baking tray with baking paper. Put croque on top. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 10-12 minutes. Cut each baguette in half again and serve immediately.