With the rack in the middle position, preheat the oven to 180°C (350°F). Butter and flour 12 muffin cups.
In a saucepan over medium heat, soften the beets in the butter and lemon juice, about 5 minutes. Let cool partially. Refrigerate until no longer warm to the touch.
In a bowl, combine the flour, baking powder and salt.
In another bowl, beat the eggs, sugar, lemon zest and vanilla with an electric mixer for about 2 minutes. With the mixer on low speed, add the dry ingredients, alternating with the beet mixture.
Divide the batter among the muffin cups. Sprinkle with the slivered almonds. Bake between 20 and 22 minutes, until a toothpick inserted in the centre of a cake comes out clean.
Loosen the cakes by running a knife around the inside of each muffin cup. Unmould gently, without inverting, and place on a cooling rack. Let cool completely.
In a small saucepan, bring all the ingredients to a boil, stirring constantly. Reduce the heat and simmer until slightly syrupy, about 10 minutes. Strain. Discard the beets. Let cool partially and refrigerate until well chilled.
In a bowl, whip all the ingredients with an electric mixer until stiff peaks form.
Spread about 45 ml (3 tablespoons) mascarpone cream on each plate. Place a cake in the centre. Drizzle with beet syrup. Garnish with slivered almonds.
Serve with Lemon Balm Tisane.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.