Preheat the oven to 375 degrees F; butter a shallow 3-quart baking dish. Melt 6 tablespoons butter in a large pot or Dutch oven over medium-high heat. Add the onions and celery and cook, stirring occasionally, until softened, about 5 minutes. Add the mustard greens and cook, stirring, until wilted, about 3 minutes. Stir in the ham, thyme and sage. Add the broth, 1 teaspoon salt and a few grinds of pepper; bring to a simmer, then remove from the heat.
Whisk the eggs in a large bowl. Add the bread cubes and ham-broth mixture and stir until combined. Transfer the stuffing to the prepared baking dish. Cut the remaining 2 tablespoons butter into small cubes and scatter evenly over the stuffing. Cover with foil and bake 30 minutes, then uncover and bake until golden, 20 to 30 more minutes.
Photograph by Ryan Dausch