Preheat oven to 375 degrees Fahrenheit.
Gently toss peaches and sugar in large bowl and let sit for 30 minutes, tossing occasionally.
Drain peaches of their liquid and reserve 3 tablespoons of the peach juice; discard the remaining juice.
Combine blueberries with peaches.
Whisk reserved peach juice, cornstarch, lemon juice, lemon zest, salt, and cinnamon over peaches and blueberries/rhubarb.
Mix everything together and pour into an 8- inch square baking dish.
You can substitute 2-3 stalks of rhubarb instead of blueberries for a tangy variation.
While peaches are macerating, mix all dry ingredients together.
Add in butter/margarine or oil and mix until the mixture clumps into crumbly balls. Pour over fruit.
Bake the crisp for 35-45 minutes or until the topping is browned and the fruit is bubbly around the edges.
You can subsitute margarine, or for a healthier alternative, 5-6 tablespoons oil, for the butter.