Preheat the oven to 175 degrees Celsius.
Cut the rhubarb into 3-4 cm long pieces. Place in an oven safe baking pan.
Drizzle with lemon juice, grate some lemon zest and sprinkle the granulated sweetener. Stir to combine. Bake in the oven for 15-20 minutes, until tender and a bit caramelized. The rhubarb should be tender, yet still firm and not soggy.
When the rhubarb is ready take it out from the oven, save 4-6 pieces for serving.
Place the rest of the roasted rhubarb stems in a pot, add about 2 dl of water and vanilla powder. Bring to a boil, then lower the heat and let it simmer for a few minutes.
Meanwhile add the ricotta to a bowl and mix using an electric balloon whisk until fluffy and smooth. Place in the fridge.
Mix the rhubarb and water using a hand mixer until you have a smooth textured cream/soup. Place back on the stove for a few minutes. Add more water if needed. Taste to check if more sugar or vanilla is neded.
Preheat a nonstick pan over medium heat. When the pan it hot add the slivered almonds and dry roast until golden brown. Make sure to stir regularly and keep an eye on them so they don't burn.
Divide the soup between bowls, top with whipped ricotta, dry roasted almonds, roughly chopped almonds, rhubarb stems and fresh basil.