Mix the filling. Preheat an oven to 400°F (205°C). In a large bowl, mix together 2 lbs sausage, 3 Tbsp Dijon mustard, 2 Tbsp sage, 2 Tbsp rosemary, 2 tsp lemon zest, 1 tsp mace, 1 tsp kosher salt, and ½ tsp black pepper until very well blended.
Cook the veggies. Heat 2 Tbsp olive oil in a large pan over medium heat, then add sliced leeks, onion, and carrots. Season lightly with salt and pepper, then cook, stirring often, until softened, about 4-6 minutes. Use a spatula to clear the center of the pan, pushing the veggies towards the sides.
Add sausage. Raise the heat to medium-high, add all of the sausage to the pan, then cook it alongside the veggies, stirring occasionally, until browned on all sides and mostly cooked through.
Deglaze with wine. Pour in ½ cup (118ml) white wine. Use a spatula to scrape any browned bits off the bottom. Allow the wine to reduce by half, then sprinkle in 1 Tbsp (8g) flour. Stir will.
Add spinach. Stir in an entire bag of baby spinach, cook until wilted, then turn off the heat. Taste and adjust seasoning if needed.
Top with phyllo. Working with one piece of defrosted phyllo dough at a time, either cut or break it into small pieces (roughly 3-inch squares, but don’t worry if the phyllo breaks), then crumble and place on top of the filling with the edges facing up. Repeat until the filling is completely covered with phyllo – it’s ok to really pack it in!
Bake. Brush melted butter lightly all over the phyllo, then sprinkle with sesame seeds. Bake for about 30-40 minutes, until the phyllo is golden brown and impossibly crispy. Enjoy!