Place the berries, sugar and lemon juice in a small saucepan and place over medium heat, bring to a boil and cook for 5 minutes. Remove from heat and sieve to remove seeds.
Meanwhile, dissolve gelatin in cold water and let it swell for about 5 minutes. Add the gelatine over the warm raspberry sauce and stir to dissolve. Let the raspberry mixture cool at room temperature.
Whip the chilled whipping cream until stiff peaks form. Add the cooled raspberry mixture and mix until well combined.
Fill the mixture into the prepared tart pan, smoothing out evenly, and refrigerate for at least 4hours or better overnight to firm up.
When the mousse is chilled and firm, you can remove the tart by gently pressing against the removable bottom of your pan. Decorate the tart with fresh berries or other toppings, serve chilled.