Pour the heavy cream into a medium saucepan and bring to a simmer.
Once the cream comes to a simmer add the lemon juice and whisk until the cream begins to thicken, roughly 13-15 minutes.
When thickened, turn off the heat and transfer the pan to an ice bath. Allow to cool for 10 minutes.
Place the lined sieve inside a slightly larger bowl and transfer the cooled cream to the lined sieve.
Cover the cream and store in the fridge for 24 hours.
After 24 hours the cream will set and you will get a little bit of whey from the cream into the bowl that you placed below the strainer. Save this to use for other recipes. What's left in the cheesecloth will be a thick mascarpone cheese.
Butter a slice of bread, then sprinkle sugar on top and broil on high on the second rack from top for 2 minutes.
To assemble, spread the mascarpone cheese on top, add a dollop of raspberry jam, and garnish with fresh raspberries.