In a large, heavy-bottomed Dutch oven, heat 1 tablespoon olive oil over medium-high heat. Add the sausages and sear on all sides until golden brown, 8 to 10 minutes. Remove from the pot and set aside.
Add the rest of the olive oil to the pot, turn the heat down to medium and add in the onions, carrots and celery. Add a pinch of salt, which will help draw some moisture out of the vegetables and keep them from browning too much. Cook gently, stirring often, until slightly softened, about 5 minutes, being sure to scrape up all the caramelized sausage bits on the bottom of the pot.
Add the garlic and rosemary and continue to sweat until the garlic is fragrant and translucent, about 5 minutes more. If the vegetables begin to brown, turn down the heat.
Add in the sausages, potatoes and plum tomatoes, breaking apart the tomatoes with a wooden spoon or your hands as they go into the pot. Fill the empty tomato can halfway with water, then add this water to the pot. Stir well and let simmer for 15 to 20 minutes, until the potatoes are cooked through, adding a few tablespoons of water if the sauce begins to look dry. Add in the butter beans and stir gently so as not to break up the beans. Taste the casserole and add salt, pepper, sugar and red pepper flakes to taste.
Just before serving, add in the spinach and stir until just wilted. Plate into individual bowls and garnish with basil and lemon zest. Serve with the Rosemary and Cheddar Soda Breads.
Rosemary and Cheddar Soda Breads:
Preheat the oven to 400 degrees F. Lightly oil a baking sheet or line with parchment paper.
Sift the flours, baking soda and salt together in a bowl. Stir in the Cheddar and chopped rosemary. Make a well in the middle. Add the buttermilk and, using a fork, mix until just combined. This will give you a soft, sticky dough, which makes a nice moist loaf. With floured hands, divide the dough into 4, scrape each one onto the prepared baking sheet and pat into a light round. Using a chef's knife, deeply score each soda bread with a cross on top (this will allow the heat to penetrate the center of the dough).
Bake until the breads are dark golden all over and sound hollow when you tap the bottoms, about 30 minutes.