Chard gratin

Adapted from French Taste by Laura Calder
serves 4
bechamel:

1 1/2 cups milk
1/4 onion
1 bay leaf
1 garlic clove
2 tbsp butter
2 tbsp flour
1/4 cup sour cream
gratin:

1 bunch chard
1 tbsp olive oil
1 onion, sliced
1 ham steak, chopped (optional)
100 g Gruyere, grated
Finely grated Parmesan to taste