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1
Slice 1 garlic clove in half crosswise and set aside; smash remaining garlic cloves with the flat side of a chef's knife.
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2
Heat 4 Tbsp. oil in a large Dutch oven or other heavy pot over medium. Cook smashed garlic, stirring often, until golden, about 5 minutes. Add onion and fennel and season lightly with salt. Cook, stirring often, until onion is translucent and fennel and onion are browned around the edges, 8-10 minutes. Add lemon zest, rosemary, and 3/4 tsp. red pepper flakes and cook, stirring often, until rosemary is very fragrant, about 3 minutes. Add bay leaves, beans, feta brine (if using; anywhere between 1/2-1 cup is fine), a couple generous pinches of salt, and 8 cups water and bring to a simmer. Partially cover pot, reduce heat to low, and gently simmer stew until beans are creamy and tender all the way through, 60-70 minutes.
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3
Add basil sprigs and olives to stew, then add escarole in batches, letting wilt slightly before adding more; simmer just until escarole is tender, about 3 minutes. (If stew is too thick, add more water to reach desired consistency.) Stir in lemon juice; taste and season with more salt if needed. Pluck out bay leaves; partially cover pot and keep stew warm over low while you make the toast.
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4
Heat broiler. Drizzle 2 Tbsp. oil total over both sides of bread and place on a rimmed baking sheet; season with salt. Broil until bread is golden and toasted, about 2 minutes per side. Let cool slightly, then rub with the cut sides of reserved garlic clove.
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5
To serve, slice toast in half and divide among shallow bowls; ladle stew over. Top with feta, basil leaves, and more red pepper flakes and drizzle with oil.
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6
Stew can be made 3 days ahead. Let cool; cover and chill. Reheat covered over low.
