Preheat the oven to 180C/350F. Grease six 3/4 cup ramekins and set aside.
In a small saucepan, combine 1/2 cup of the allulose with the water. On low heat, bring the mixture to a simmer. Once it beings to simmer, increase to medium heat. While stirring regularly, allow it to thicken and become an amber brown color. Remove from the heat and distribute amongst each of the ramekins, brushing around the sides to coat them in the warm sugar liquid.
In a separate saucepan, combine the milk with the remaining sweetener. On low heat, bring to a gentle simmer. Remove from the heat and whisk in the eggs, vanilla extract, and salt, until smooth and combined. Strain the mixture into the ramekins.
Place the six ramekins in a large baking dish. Pour boiling water into the dish until halfway up the ramekins. Bake for 45-50 minutes, until it is mostly set in the middle. Remove from the oven and let cool completely. Once cool, refrigerate the individual flans for at least 6 hours, or overnight.
Once the flans are set, use a slightly moist knife and cut around the ramekins and flip onto a plate.
* I used allulose. Monk fruit and erythritol don't caramelize well. TO STORE: This dessert must always be stored in the refrigerator, as it is dairy based and is served chilled. Cover the ramekins and they will keep well for up to 1 week. TO FREEZE: Cover your ramekins with plastic wrap, ensuring there are no airy pockets. Store them in the freezer for up to 2 months. You MUST thaw them in the refrigerator completely.