How to Make It
Heat oil in a large skillet over medium-high. Season pork with salt; brown all four sides, about 3 minutes per side. Add the shallots and cook until golden, about 3 minutes. Add the stock and bring to a simmer. Reduce heat; cover and cook until a thermometer inserted into thickest portion of pork registers 140°F, 8 to 10 minutes. Transfer pork to a cutting board; slice.
Add chard stems to the skillet; cook over medium-high until softened, 2 to 3 minutes. Add chard leaves; cook, stirring often, until wilted, about 2 minutes. Transfer chard to a plate, reserving cooking liquid in skillet.
Stir butter, lemon juice, and red pepper into the pan. Cook, whisking occasionally, until pan sauce is smooth and slightly thickened. Serve over the pork and chard.