Bette à carde gratiné (Swiss Chard Gratin)

2 bunches chard leaves, chopped (about 8 cups packed or 2lbs)
1 cup water
1 Tbsp vegetable oil
1 Tbsp (½ oz/14gr) butter
1½ cups milk
3 Tbsps flour
1 Tbsp + 2 tsps wholegrain mustard 
1½ cups grated Gruyère (6.3oz/180g)
Sea salt and black pepper
¼ cup grated Parmesan cheese (0.8oz/22.5g)
2 to 3 Tbsps bread crumbs

SERVE WITH
Crusty bread

1. Preheat oven to 350°F. 
2. Bring a  saucepan of salted water to boil. 
3. Thinly slice the reserved chard stems, add them to the water, and cook for 2 minutes. Add the leaves and continue cooking until tender, about 3 to 4 minutes longer. 
4. Drain and allow to cool, then gently squeeze out the excess liquid and coarsely chop.
5. In a large pan heat oil and butter over medium heat. When butter has melted, whisk in the flour until blended and whisk until the flour cooks, or starts to brown. 
6. Slowly whisk in the milk. Continue cooking and stirring until the sauce thickens, 3 to 5 minutes. Season with salt and pepper and stir in the mustard and the Gruyère. 
7. Stir in the cooked chard and transfer to a buttered gratin dish. 
8. Sprinkle with the Parmesan and breadcrumbs. 
9. Bake for 20 minutes or until hot and bubbling. 
10. Put under the broiler to get the top brown and crispy.
11. Put the hot potatoes onto a plate and spoon the gratin over them.
12. Serve immediately.