Arrange oven racks in top and bottom third of oven to accommodate two trays.
On a large-rimmed baking sheet, coat potatoes with oil, sprinkle with salt, and prick each potato a few times with a knife. Set aside until the beans are ready to go in the oven along with the potatoes.
In a large pot or dutch oven, heat oil over medium. Add onion and sauté until beginning to brown (about 15 minutes); remove from heat. Add remaining ingredients except beans, whisking to combine; mix in beans. Keep in the pot or dutch oven and cover with a lid, or transfer mixture to a large ovenproof dish and cover tightly with foil.
Add beans to bottom rack of oven and potatoes to top rack of oven.