Heat oven to 350F.
Make the Bread Crumbs: Melt 2 tablespoons of butter and toss it with the bread crumbs. Spread out the bread crumbs on a baking sheet and bake for 8-10, stirring occasionally, until golden brown.
Prepare the Greens: Wash the greens you are using well to make sure to rinse off all the dirt.
For chard, fold each leaf in half and stem the chard, cutting the stem off of the leaves. Cut the leaves into 1/2 inch slices and also chop up the top half of the stem (discarding the tough bottom part). Reserve the chopped chard stems for the next step.
If using other types of greens, cut off the stems if they are long and then cut the leaves in 1/2 inch slices.
Saute the Greens: In a large skillet, melt the remaining 2 tablespoons of butter over low heat.
Add the reserved chard stems, shallots, garlic, and salt. Gently saute the onion and garlic until soft and translucent, about 10 minutes.
Add the chopped greens, cover the skillet and steam for 15 minutes, or until the greens are tender and wilted.
Add the flour to the skillet and stir to combine. Then pour the milk over and bring to a simmer over medium heat. Simmer for 5 minutes to thicken the milk.
Bake the Gratin: If you are using an oven safe skillet, leave the greens in the pan. If not, transfer the mixture to a casserole dish. Either way top the greens with the toasted bread crumbs and then sprinkle the parmesan on top.
Bake for 20-25 minutes, until browned and bubbling. Let cool for 5 minutes and serve.