Combine stock and dried porcini in a medium saucepan over medium heat. Bring to a gentle simmer, then reduce heat to low. Let porcini steep for 10 minutes. Remove porcini with a slotted spoon, roughly chop, and set aside. Keep stock warm on the back burner.

Heat 1 tablespoon butter and the olive oil in your large pot over medium-high heat. Add cremini mushrooms in a single layer and cook undisturbed for 4-5 minutes until deeply browned on the bottom. Stir once, cook 2 minutes more, then transfer to a plate. Season lightly with half the salt.

Reduce heat to medium. Add remaining 2 tablespoons butter to the same pot. Add onion and cook, stirring occasionally, until soft and translucent, about 5 minutes. Stir in arborio rice and cook for 2-3 minutes, stirring constantly, until the edges of the grains turn translucent and you smell a faint toastiness.

Pour in white wine and stir until completely absorbed, about 1 minute. Add the chopped porcini and a ladleful of warm stock. Stir gently but constantly, keeping the liquid at a lazy simmer. When the stock is nearly absorbed, add another ladleful. Continue this rhythm for 18-22 minutes, tasting as you go. The rice should be tender with a slight firmness at the center (al dente) and the dish should flow like lava, not stand in a mound.

Remove from heat. Stir in the seared cremini mushrooms, grated Parmigiano-Reggiano, thyme, remaining salt, and pepper. The residual heat should make it creamy and loose. If too thick, loosen with a splash of warm stock. Cover and let rest for 2 minutes. Serve immediately in warm bowls with extra cheese on the side.