Place the almonds and hazelnuts in a 320°F/160.0°C oven to warm them.
Combine the sugar and water in a saucepan and boil to soft ball stage (239°F/115.0°C).
Remove from the heat and add the warmed nuts. Mix continuously with a heatproof spatula until the nuts are completely covered with sugar. (Because of the movement, the sugar will crystallize and stick to the nuts, forming a nice coating.)
Return to the heat to begin the actual roasting process. Set the heat at a low temperature, and stir the nuts constantly with the heatproof spatula to allow the sugar to caramelize without burning. At first, the sugar around the nuts will be crystallized and grainy. The heat will cause the sugar to melt and caramelize. Cooking time varies, but the caramelization is complete when the sugar is completely melted around the nuts and the nuts are shiny.
Remove from the heat and pour the nuts onto a Silpat. Spread into a single layer and separate as they cool.
When cool, place the nuts in a food processor and grind until the mixture has a runny consistency.
Add the nut paste to the couverture and stir until combined.
If the consistency is too thin or the temperature is too warm, temper the gianduja on a marble until it reaches the correct consistency.
Use immediately or store in an airtight container in a cool, dark place.