Begin by rinsing and trimming the fresh green beans to remove the ends. Set aside and heat a large skillet over medium-high heat.

Add the unsalted butter to the skillet and let it melt. Toss in the chopped red onion, stirring for 2-3 minutes until translucent and fragrant.

Introduce the sliced cremini mushrooms to the skillet, stirring gently and cooking for an additional 4-5 minutes until softened.

Add the trimmed green beans to the skillet. Season with kosher salt and black pepper, then pour in the chicken stock.

Increase the heat to medium-high and bring the mixture to a gentle simmer, stirring occasionally for 10-15 minutes.

Monitor the sauce consistency and adjust the heat as needed to ensure a desirable texture.

Taste the dish and adjust seasoning before removing from heat, ready to serve warm.