Preheat oven to 325 degrees F. Butter a 9x1 1/2-inch round cake pan. Line the bottom of the pan with parchment; set aside.
In a medium bowl combine flour, cornmeal, ground almonds, baking powder, and salt; set aside.
In a large mixing bowl beat butter with mixer on medium to high for 30 seconds. Add 1/2 cup of the honey; beat 3 minutes, until fluffy. Slowly beat in eggs, one at a time. Add three-fourths of the flour mixture; mix just until combined. Mix in milk and remaining flour just until combined. Pour batter into prepared pan.
Bake for 30 to 35 minutes, just until a wooden toothpick inserted near center comes out clean. Cool on a wire rack for 5 minutes. Remove cake from pan.
Meanwhile, in a small saucepan heat remaining 1/4 cup honey and the orange juice, whisking to combine. Poke warm cake with a wooden toothpick. Brush on honey-orange juice until absorbed. Cool cake completely.
For topping, just before serving, in a medium mixing bowl whip mascarpone on medium to high for 30 seconds. Add cream and 2 Tbsp. honey. Beat just to soft mounds. Spoon topping on cake, drizzle with additional honey, then top with grapefruit, orange, and blood orange slices and chopped almonds. Sprinkle lemon thyme over fruit segments, if desired.