In a food processor combine drained cannellini beans, 1/4 cup of the bread crumbs, the onion, walnuts, basil, and garlic. Cover and process until mixture is coarsely chopped and holds together.
Shape the bean mixture into four 1/2-inch-thick patties. Place the remaining 1/2 cup bread crumbs in a shallow dish. Carefully brush both sides of each patty with oil. Dip patties into bread crumbs, turning to coat.
Preheat a grill pan or large skillet over medium heat. Add patties to pan or skillet. Cook for 10 to 12 minutes or until heated through, turning patties once halfway through cooking. (Reduce heat to medium-low if patties brown too quickly.)
Spread cut sides of bun bottoms with ranch salad dressing. Top with burgers, spinach, tomato slices, and bun tops.