Preheat the oven to 400°F. If starting with fresh Hatch green chiles rather than already roasted peppers, arrange the 4 chiles on a baking sheet. Roast for approximately 20 minutes, turning once or twice during cooking, until the skins are blistered and blackened in spots. The chiles may also be prepared on a grill using the same visual cues for doneness.
Transfer the hot roasted chiles immediately to a sealable plastic zip-top bag. Close the bag and allow the peppers to steam for 20 minutes to loosen their skins.
When the chiles are cool enough to handle, remove the blistered skins and stems. Open each chile and rinse away the seeds. Set the prepared peppers aside for filling the steaks.
Arrange the thinly sliced sirloin pieces in a single layer on a rimmed pan.
Distribute the steak seasoning evenly over the meat, then season lightly with salt and ground white pepper.
Cover each steak with a portion of baby spinach and strips of the roasted Hatch green chiles.
Add a piece of queso fresco near the center of each steak, dividing the 2 oz filling portion among the rollups.
Beginning at one end of each steak, roll the meat firmly around the spinach, chile, and cheese filling, keeping the ingredients contained as tightly as possible.
Secure each steak roll with toothpicks wherever necessary to prevent the filling from slipping out while the meat cooks.
Arrange the secured steak rollups on a rimmed baking sheet. Place the pan on the center rack of the 400°F oven.
Roast for approximately 15 minutes, or until the exterior of the steak begins to brown and the queso fresco inside starts to soften.
Step 5: Prepare the Cilantro-Lime Yogurt Sauce
While the steak rollups are in the oven, place the Greek yogurt, fresh cilantro, lime juice, and lime zest in a small blender.
Pulse the mixture several times, stopping when the cilantro is evenly dispersed but remains visible in small pieces rather than being completely puréed.
Season the sauce with salt and ground white pepper according to taste.
Take the roasted steak rollups out of the oven and leave them to rest for 5–10 minutes before slicing.
Cut the rested rolls crosswise into portions and serve them with a generous drizzle of the cilantro-lime yogurt sauce.
Finish each serving with the remaining 2 oz of crumbled queso fresco.