1 pound fresh baby spinach leaves, washed
6 tablespoons unsalted butter, divided
1 1/2 pounds boneless, skinless chicken breasts or thighs, chopped into 1-inch chunks
Kosher salt and freshly ground black pepper
5 medium cloves garlic, minced
2 tablespoons all-purpose flour
1/2 cup homemade chicken stock or low-sodium broth, plus more as needed
1/2 cup half-and-half
6 ounces feta cheese, crumbled
3 scallions, white and light green parts only, chopped
1 small bunch dill, chopped, plus more for garnish
6 to 8 sheets phyllo dough, thawed and covered with a towel