Put the pancetta and a small glug of oil in a large saucepan and heat over medium heat. Cook, stirring occasionally, until the pancetta is browned (it does not have to be crisp), about 5 minutes, Stir in the onion, celery, and garlic. Increase the heat slightly and cook, stirring occasionally, until the vegetables are browned, about 5 minutes.
Add the wine and increase the heat to high. Bring to a boil, scraping up the browned bits in the pot with a wooden spatula. Add the clams, cover and cook, occasionally shaking the pot, until the clams have opened, 3 to 5 minutes.
Drain 1 can of the beans and add to the pot. Add the second can of beans with their liquid and the thyme. Reduce the heat to medium-low and simmer for 5 minutes. Cover, remove from the heat, and let stand for 5 to 10 minutes. Discard any unopened clams.
Ladle the soup and clams into bowls. Garnish each with a few pinches of lemon zest, a sprinkle of scallions, a few grinds of pepper and some flaky salt. Serve hot with the crostini (toasted French bread slices drizzled with olive oil).