How to Make It
In a large nonreactive saucepan, heat the oil. Add the fennel and onion; cook over moderate heat, stirring, until the onion is translucent and beginning to brown, about 5 minutes. Add the garlic; cook, stirring, until fragrant, about 1 minute. Blend in the tomato paste. Add the stock and the tomatoes with their juices. Season with salt and pepper; bring to a simmer. Reduce the heat, cover and simmer gently for 20 minutes.
Meanwhile, heat a large heavy skillet. Prick the sausages all over with a fork and add to the pan. Cook over moderately low heat, turning, until browned and cooked through, about 20 minutes. Drain on paper towels and let cool slightly. Halve the sausages lengthwise, then cut them into 1/3-inch pieces.
Bring the soup to a boil, add the orzo and cook, stirring occasionally, until al dente, about 10 minutes. Add the spinach and sausages and cook, stirring, until the spinach is wilted and the sausages are heated through, 2 to 3 minutes.