Prepare the Madeira jelly according to the sachet instructions, using only 75 cl of water.
Cook the quail eggs for 3 minutes in boiling water for a soft texture, then plunge into cold water and peel. Pour a thin layer of jelly into the base of a savarin mold and refrigerate until set.
Halve the cucumber lengthwise and cut into thin strips with a paring knife. Blanch the strips for 3 minutes in boiling salted water, refresh under cold running water, and drain thoroughly.
Once the jelly layer has set, arrange in successive layers: 10 quail egg halves (cut side down), cucumber strips, and smoked salmon slices, interspersed with tarragon and dill leaves.
Pour over a little more jelly, add the remaining salmon, cucumber, and eggs. Season with freshly ground white pepper, top up with the rest of the jelly, and chill for at least 4 hours.
Whip the cream to Chantilly consistency, gradually folding in the horseradish sauce. Season with salt and pepper, then refrigerate until serving.
To serve, briefly dip the mold base in lukewarm water, unmold onto a platter, spoon the cream into the center, and garnish with salmon roe.
