In a sauté pan over medium heat, melt 3 tbs. of butter. Add 1 cup of leeks and cook until tender, but not browned, about 5 minutes.
Add the cognac and simmer for 1 minute. Then add the lobster meat and cook for 1 to 2 minutes. Remove from heat.
In a saucepan heat the chicken stock.
In another sauté pan melt 1 tbs. butter and 1 tbs. olive oil add the fennel and cook until tender (about 5 minutes). Add the remaining ½ cup of leeks and cook 2 minutes. Then add the garlic and cook another 2 minutes.
Stir in the rice and cook for 3-4 minutes. Add the wine and cook until absorbed.
Add the stock ½ cup at a time, cooking until each addition is absorbed before adding next amount. Finally add mascarpone cheese, stirring until creamy, about 3 minutes.
Remove from heat and stir in lobster meat, lemon juice and zest, salt, pepper and chervil.