Meanwhile, heat a large non stick skillet over medium high heat. Add the mushrooms and spread them evenly over the pan in a single layer. Cook, undisturbed, for about 3 minutes, or until the bottoms have browned. Toss the mushrooms and sprinkle with salt. Continue to cook another 4-5 minutes or until evenly browned, stirring occasionally.
Add the butter to the skillet, followed by the garlic. Stir until butter is melted and garlic is fragrant, about 1 minute. Add the cream, and bring the mixture to a boil, then lower the heat to maintain a gentle simmer. Cook, stirring occasionally, for 4-5 minutes, or until cream is thickened. If you run your spatula through the sauce, it should take a second or two to run back together.
Add the lemon juice and thyme leaves and stir. Taste, then add additional lemon juice, salt, or pepper as desired. Spoon the mixture over the toast and top with the chives. Serve immediately.