In a saucepan, off the heat, combine the sugar and cornstarch. Add the cherries, strawberries, lemon juice, and water. Stir to combine. Bring to a boil over medium heat and simmer gently for about 15 minutes, stirring frequently. If necessary, skim off the surface during cooking. Let cool. Cover and refrigerate until the mixture is room temperature.
With the rack in the middle position, preheat the oven to 190 °C (375 °F).
In a food processor, combine the flour, sugar, and salt. Add the butter and pulse for a few seconds at a time until it has the size of peas. Add the water and oil. Pulse again until the dough just begins to form. Add water, if needed. Form a cylinder with your hands and cut into 12 slices.
Place the dough into twelve 7.8 x 2-cm (31/16 x 7/8-inch) aluminum tart pans and press the dough with your thumbs. Place on a baking sheet and bake for about 18 minutes or until the crust is golden brown. Let cool. Unmould.
In a bowl, whip the cream with the sugar and vanilla until stiff peaks form. Refrigerate, if necessary.
Fill the tartlets with the filling. Top with the whipped cream and garnish with the cherries and strawberries. Store in the refrigerator.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.