Preheat the oven to 400°F (200°C).
In a large skillet, melt the butter over medium heat. Add the diced mushroom stems and onions, sautéing until the onions become translucent, about 10 minutes.
Stir in the minced garlic, cooking briefly until fragrant. Remove the skillet from heat.
Incorporate the breadcrumbs and grated Romano cheese into the skillet, stirring until evenly combined. Season with salt and pepper to taste, being cautious with the salt as the cheese adds a natural saltiness.
Using a teaspoon, fill each mushroom cap with the mixture, gently mounding extra filling on top to form a small peak.
Arrange the stuffed mushrooms on a baking sheet lined with parchment paper or foil.
Bake in the preheated oven for 15-20 minutes, or until the tops are golden and crispy.