Preheat oven to 400°F. In a baking dish, place tomatoes cut sides up. Mist with cooking spray and season with salt and pepper. Roast until tender and slightly shriveled, about 30 minutes.
Meanwhile, place a heat-proof bowl over a pot of simmering water; in bowl, whisk together egg yolks, yogurt and lemon juice. Cook, whisking often, for 5 to 8 minutes or until thick. (NOTE: mixture will appear thick at first, then will thin out and thicken again.) Remove bowl from heat and whisk in 4 tsp butter and mustard until smooth. Season with additional salt. Cover and set aside.
In a large skillet on medium high, heat remaining 1 tsp butter. Add mushrooms and cook, stirring, until softened, 3 to 4 minutes. Add spinach and cook, stirring, until spinach is wilted, 1 to 2 minutes. Remove from heat, cover and set aside.
Poach eggs: Fill a large, shallow saucepan with 3 inches water; bring to a boil. Add vinegar and reduce heat to medium-low to maintain a gentle simmer. In a small dish, break 1 egg and gently slip into simmering water. Repeat with remaining eggs and cook for 3 to 4 minutes, or until desired doneness. (NOTE: It may be easiest to do this in 2 batches of 4 eggs each.)
Divide spinach-mushroom mixture among plates and nestle 2 baked tomato halves on top. Top each tomato with a poached egg, then spoon sauce over top and sprinkle with chives.
Serving Size: 2 eggs and 1/4 of spinach mixture, tomatoes, and sauce
You can put together your own combination of preferred mushrooms or look for already-combined sampler packages at your local market (TRY: Chef's sampler from Mycopia, a California grower). And don't be afraid to break up this recipe: Try the sautéed side over top of pan-seared chicken, polenta or even pizza.