Heat a medium nonstick skillet over medium-high heat. Add 2 tablespoons of butter and the olive oil, letting the butter melt. Add the mushrooms (the pan will be full but they'll shrink in a minutes) and carefully stir. Saute over medium-high heat for about 3 minutes, until the mushrooms start to shrink down a bit. Season with salt and pepper, and add the thyme.Add the garlic and the remaining tablespoon of butter. Keep cooking for 6 to 8 more minutes; the mushrooms will get very juicy for a little bit, giving up all their liquid. Keep cooking them until that watery liquid evaporates and you just have a little buttery juice left.Sprinkle in the flour and stir well with a whisk. Cook for one minute, stirring. Whisk in the stock until smooth, then reduce heat and simmer for 3 to 5 minutes, until the gravy has thickened a little. Whisk in the half-and-half and simmer for another minute.Taste to adjust seasoning. I like it with about a tablespoon of Worcestershire and a dash of Tabasco.