Combine the water, lemon juice, sugar, and 1 tsp. of salt in a pot and bring to a boil. Add the mushrooms and boil until just tender, about 3 to 4 minutes. Drain the mushrooms well and place in a bowl; add the oil, pesto, hot pepper sauce, and bell pepper and toss to coat.
Cool the mushrooms to room temperature; cover and chill in the fridge for at least two hours. Season with salt and pepper to taste before serving.