Grilled Flatbread

Dissolve yeast in 3 cups warm water in a large bowl. Add all-purpose and whole wheat flours; mix with your fingertips until a shaggy dough forms. Cover bowl with plastic wrap and let rest at room temperature for 20 minutes.
Sprinkle salt over dough, then add sour cream; knead until well incorporated and dough pulls away from sides of bowl and holds together in a loose, wet ball, about 5 minutes (dough will be very soft and wet; lightly moisten your hands to prevent sticking if needed). Cover bowl with plastic wrap and let dough rise at room temperature for 30 minutes.
Knead dough an additional 4-5 times to deflate. Cover and chill for up to 2 days. (Dough will develop in flavor and continue to rise slowly in refrigerator.) Alternatively, let dough stand at room temperature until doubled in volume, 3-4 hours (the warmer and more humid your kitchen is, the faster it will rise). Chill for 1 hour before grilling to make it easier to handle.
Build a medium-hot fire in a charcoal grill, or heat a gas grill to high. Divide dough into 8 equal portions. Generously flour a work surface. Working with 1 or 2 portions at a time (depending on how many flatbreads will fit on your grill), roll out dough or press with your hands into a 1/4"-thick shape. (It doesn't have to be perfectly round.)
Brush grill rack with oil. Grill flatbreads until lightly charred on one side and no longer sticking to grill, 2-3 minutes. Using tongs, turn flatbreads and grill until cooked through, 1-2 minutes longer. Add desired toppings and serve warm.