Step 1

In a stock pot or large Dutch oven, melt butter over medium-high heat. Add leeks and cook, stirring occasionally, until soft and lightly golden, 5 to 7 minutes. Stir in garlic and red-pepper flakes (if using), and cook until fragrant, about 1 minute. Add cabbage, salt and pepper, and cook until cabbage wilts slightly and begins to caramelize, about 10 minutes.

Step 2

Stir in stock, rice and thyme. If using the Parmesan rinds, add them now, and bring everything to a simmer. Cook, partly covered, over medium-low heat until cabbage and rice are very tender, 20 to 25 minutes.

Step 3

Discard thyme sprigs and Parmesan rinds. Using a Microplane or other fine grater, grate in the zest from the lemon. Stir in grated Parmesan and dill.

Step 4

Halve the lemon and squeeze in the juice from half of it. Cut the remaining lemon half into wedges for serving. Taste soup and add more salt and pepper, if needed. To serve, ladle into soup bowls, and garnish with more grated cheese, a lemon wedge and more red-pepper flakes, if you like.