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1
To make the main spice blend: Finely grind the bay leaves, fennel seeds, and cumin seeds together and immediately mix with the cayenne.
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2
To make the beans: Place the beans in a large container, add enough cold water to cover by 2 inches, and cover tightly. Refrigerate for at least 12 hours and up to 16 hours. Drain well, rinse, and drain again.
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3
Heat a very large Dutch oven or other heavy pot over medium-high heat. Add enough olive oil to cover the bottom (4 to 5 tablespoons) and heat until very hot, shimmering, and almost smoking, about 2 minutes. Add the spinach and quickly spread it in an even layer. You want a really nice char on the spinach. Sear, using a wooden spoon to stir and smush the spinach and scraping the pot as needed. Keep cooking and adding more olive oil (up to 4 tablespoons more) if the spinach threatens to burn. The spinach is done when it's a bunch of crisp green bits and the olive oil turns dark green, which should take about 10 minutes.
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4
Add the onion, stir well, and season generously with salt and half the spice blend. Cook, stirring, for 2 minutes. Add the vinegar and stir well. Add the drained beans and the remaining spice blend. Stir well and add enough water to cover (3 to 4 cups).
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5
Raise the heat to high, cover, and bring to a simmer. Reduce the heat to maintain a simmer and cook until the beans are tender, 40 to 60 minutes. Remove from the heat and season to taste with salt. Serve hot, warm, or at room temperature.
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6
Make-ahead: The beans can be cooled completely in their cooking liquid and then refrigerated in their cooking liquid in an airtight container for up to 1 week or frozen for up to 2 months.
