mushroom stock Place porcini mushrooms in a medium bowl and cover with 1 cup boiling water; let sit 30 minutes. Line a fine-mesh sieve with 2 layers of paper towel and set over another medium bowl; pour mushroom mixture through (towels will catch any grit from mushrooms). Set aside porcini mushrooms and soaking liquid.
Heat oil in a large saucepan over medium heat. Cook onion, carrot, and celery, stirring occasionally, until softened, 5–7 minutes. Add crimini mushrooms, parsley, thyme, peppercorns, salt, reserved porcini mushrooms and their soaking liquid, and 6 cups cold water. Bring to a boil, reduce heat, and simmer 30 minutes.
Strain stock into a clean saucepan and keep warm over low heat (you should have about 6 cups).
DO AHEAD: Stock can be made 4 days ahead. Cover and chill. Reheat before using.
Paella and assembly 6 tablespoons olive oil, divided 1½ pounds crimini mushrooms, quartered Kosher salt and freshly ground black pepper 1 large onion, very finely chopped ½ red bell pepper, very finely chopped ½ poblano chile, seeds removed, very finely chopped 4 cloves garlic crushed, chopped to a paste 1½ cups short-grain Spanish rice (such as Bomba)