Put the stock in a saucepan and sprinkle over the gelatin. Let sit 5 minutes to soften. Gently heat, stirring to dissolve. Remove from the heat and stir in the Cognac. Add salt and pepper, to taste. Cool. Pour the thinnest layer in the bottom of each of 4 ramekins. Refrigerate until sticky, but not completely set. Keep the remaining consomme mixture out.
While you wait for the ramekins to set, put the eggs in a pot and pour in cold water to cover. Bring to a boil, uncovered. Turn off the heat, cover and set aside 8 minutes. Drain and rinse under cold water until cool. Peel and halve lengthwise.
When the stock in the ramekins is more or less set, decoratively lay the tarragon leaves on top with their best side facing down. Set an egg half, yolk-side against the tarragon, on top in each. Pour over the remaining stock. Refrigerate until fully set.
Turn the jelled eggs out onto plates and serve with a bit of rocket and buttered toast triangles.
Stir together the beef, celery, carrot, leek, tomatoes, parsley, and egg whites in a large bowl. Pour the beef stock into a pot, season with salt and pepper, and whisk in the vegetable mixture. Heat, whisking slowly, until frothy, about 10 minutes. As soon as the stock reaches a simmer, stop stirring. In a few minutes, the white mixture will have formed a firm, nasty-looking "raft" on the surface. Make a vent in the raft large enough to get a small ladle through, and continue simmering 30 minutes.
Line a strainer with cheesecloth and set over a bowl. Leaving the raft behind, ladle in the clarified consomme through the hole in the raft and let it strain through. Serve piping hot, with or without garnish.