Toast bread in a toaster.
Meanwhile, heat a large skillet over high heat. Add sugar, lemon zest and lemon juice and cook, stirring, until the sugar melts and the mixture begins to bubble, 30 seconds to 1 minute. Add strawberries and stir until juices begin to exude and the berries are heated through, 30 seconds to 1 minute more.
Spread 1 tablespoon mascarpone on each piece of toast. Top with the warm berries.