Preheat the oven to 350°F. Lightly oil a 9-inch baking dish, such as a pie plate or Pyrex dish.
In a large bowl, combine the fresh fruit with the lemon juice, sugar and cornstarch. Add baking spice, if desired. If you have time, let the mixture stand 30 minutes to macerate the fruit.
Transfer the fruit and all the juices to the dish. Crumble the topping over the fruit, leaving large clumps throughout.
Place the dish on a rimmed baking pan lined with parchment (in case there are spills!) and place in the oven. Bake crisp for 45-55 minutes or until juices bubble up around the sides and the fruit in the center of the dish is soft when poked with a fork.
Cool the crisp, then serve warm with ice cream or whipped cream. Will keep for up to 3 days refrigerated - if it lasts that long!
Sweeter fruits like berries and stone fruits will not need additional sugar, but rhubarb or tart apples will benefit from double the sugar in this formula.