Preheat a grill for medium-high heat. If you're not grilling, you can use a cast iron grill pan and heat over medium.
If using wooden skewers, soak them in water. If using metal, no prep needed.
Finely chop 1 large shallot and rinse it under cold water. Add to a medium bowl with a pinch of salt and the juice of 1 lemon. Let sit for 5 minutes.
In a large bowl, whisk together ½ cup olive oil, 2 tablespoons ground cumin, 1 tablespoon smoked paprika, and 2 teaspoons salt. Add black pepper and finely grate 8 garlic cloves into the bowl.
Tear 2 pounds of mushrooms into large 2-3” pieces and add them to the spiced oil. Toss gently to coat.
Thread the mushrooms onto skewers, inserting the skewer into the thick parts of the stems.
Stir ¾ cup whole milk Greek yogurt into the lemony shallots and season with salt and black pepper.
Grill the skewers, turning once, until charred and crisp, about 8-12 minutes.
Warm or toast the flatbreads.
Slather shallot yogurt on each flatbread, top with mushrooms, add arugula, lemon zest, lemon juice, olive oil, and flaky salt.
