Bring a medium sauce pot of generously salted water to a boil. Separate the fusilli so the noodles don’t clump together during cooking. Add the fusilli and cook, stirring occasionally, until just tender, 3 to 5 minutes. Drain the fusilli, reserving ¼ cup [½ cup] pasta cooking water. Meanwhile, prepare the remaining ingredients. 

• Cut the mushrooms into ¼-inch-thick slices.
• Trim the root ends from the leek; cut the leek in half lengthwise, then crosswise into ¼-inch-thick half-moons. Soak the half-moons in a medium bowl of cold water, stirring occasionally to loosen any dirt, and allow the dirt to settle at the bottom. Skim the leek from the surface of the water and pat dry.
• Remove any coarse stems from the broccoli leaves; cut the leaves crosswise into ½-inch-wide strips. 
• In a small bowl, stir together the cheddar, yogurt, and one-third of the mustard, adding more mustard to taste. 

In a large sauce pot over medium-high heat, warm 1 tablespoon oil until hot but not smoking. Add the mushrooms and cook, stirring occasionally, until starting to soften, 2 to 3 minutes. Add the leek, season with salt, and cook, stirring occasionally, until softened and translucent, 2 to 3 minutes. Working in batches if needed, add the broccoli leaves and smoky spice blend and cook, stirring occasionally, until the leaves are wilted and the spice blend is fragrant, 2 to 3 minutes. 

Remove from the heat and stir in the cheddar mixture, fusilli, and reserved pasta cooking water. Season to taste with salt and pepper. 

Transfer the fusilli to individual bowls and serve.