Cut ends off lemon to expose flesh, then set on 1 flat side and remove peel and white pith; discard. Cut between membranes to release segments. Remove seeds; set aside.
Heat a large skillet over medium-high. Add 3 Tbsp. butter, chard ribs, and the garlic. Cook, stirring frequently, until garlic softens, about 4 min. Add the pine nuts, and cook, stirring, until garlic is golden and pine nuts are toasted, about 1 min. more. Stems should be crisp tender. Immediately add the lemon juice, capers, and fish sauce, and cook, stirring, until lemon juice reduces slightly, 1 min. more. Stir in the lemon flesh. Scrape the stem mixture into a medium bowl. Season with salt and pepper.
Return the skillet to medium-high heat, and add the remaining 1 Tbsp. butter and the chard greens. Season with salt and pepper. Cook, tossing, until greens are just tender but still bright, about 5 min. Return half the stem mixture to the skillet, toss to combine; reserve remaining half for serving. Scrape cooked greens onto a plate.
Return the skillet to medium-high heat, and add the beans, Dijon, and 1 cup water. Using a potato masher, mash the beans until a puree forms. Continue to cook, stirring occasionally, until puree thickens, about 3 more min. Season with salt and pepper.
Spoon the puree onto the toast, and top with greens and stem mixture.
Recipe by Laura Rege.