Grind the bay leaves, fennel seeds and cumin seeds together finely, then add the ancho chile powder.
Set Instant Pot to sauté mode. Add 2 tablespoons olive oil to the bowl and heat until shimmering. Add diced onions and cook until softened and browning, 4-6 minutes. Add the mayocoba beans and 10 cups water to the bowl of the Instant Pot, then set to Pressure Cook and attach the lid, locking it into place securely. Set the pressure cook cycle for 2 minutes, then let it pressure and cook. Let the pressure vent naturally until safe to open.
While the mayocoba beans are venting, place a Dutch oven over high heat and add the remaining olive oil to the pot. Heat until shimmering. Add the spinach to the Dutch oven, smoothing out such that as many of the leaves are subjected to the olive oil and charring. Stir the leaves, adding more olive oil as necessary to make sure all of the leaves wind up charred black and the olive oil is dark green, about 10 minutes.
Drain the beans and onions into a colander, then rinse well. Add the beans and onions to the charred spinach and oil, then add half of the spice blend and salt to taste. Cook for 2-3 minutes, stirring continuously. Add the balsamic vinegar and enough water to cover the beans (3-4 cups), then cover and turn the burner on high. Once the contents of the Dutch oven are boiling, remove the cover, reduce the heat to allow the beans to simmer, then cook until beans are cooked thoroughly, 40-60 minutes.