Cook the sugar, glucose, and water in a 960-ml/1-qt saucepot over high heat.
Heat the cream, butter, and fleur de sel in a microwave, enough to warm the mixture up to about 65°C/150°F.
When the sugar turns a deep, dark caramel color, between 178 and 180°C/355 and 360°F, turn off the heat and slowly stream in the hot cream mixture, carefully whisking it in. Take great care and whisk the caramel thoroughly or it will separate while cooling.
Once cool, fill the tarts. There will be caramel left over after filling the tarts. Use it to make more tarts when necessary. This will keep for more than 6 months.