If you are using a loaf tin or similar, line the tin with cling film. If you are planning to serve individual jellies in little jars, there's no prep required.
Measure out 100 ml of smoothie and place in a pan. Sprinkle the gelatine powder over the cold smoothie, and heat gently whilst stirring until dissolved. Do not allow the liquid to boil. (See notes above if you are using gelatine leaves)
Add the remainder of the smoothie and fruit. Pour into your lined loaf tin or jars, making sure the fruit is evenly distributed.
Chill in the fridge until set (approx 2 hours if you are using frozen fruit, or 4 hours plus for fresh fruit).
If you have set your jelly in a loaf tin, tip carefully onto a serving plate and remove the cling film. If the jelly has stuck to the tin, dip in hot water briefly.
Notes
If you are making a large terrine, you'll need to use fresh fruit to achieve a firmer texture which will hold together well. If making individual summer fruit jellies, you can use either fresh or frozen fruit.You can use either gelatine powder or gelatine leaves, but be sure to follow the instructions carefully.If you want a vegetarian jelly, replace the gelatine powder with vegetarian gelatine and follow the instructions on the packet.