Line a 4-cup round mold with plastic wrap. Lay about 1/3 of the salmon over the plastic wrap, overlapping each slice slightly and hanging the ends over the edge, to cover the mold. Chop the remaining salmon into 1/2-inch pieces and set aside. In the bowl of an electric mixer, combine the cream cheese and sour cream until creamy and well combined. Add the hot pepper sauce and salt and stir well. In a small saucepan, heat the lemon juice over very low heat. Pour into a 1/2-cup ramekin and sprinkle with the gelatin, stirring until the gelatin is dissolved. Remove from the heat and stir into the cream cheese mixture. Fold in the chopped smoked salmon and scallions. Spoon 1/3 of the mousse into the mold, top with a layer of avocado slices, then gently spread with 1/2 the salmon roe. Spread 1/3 of the mousse on top of the roe, and top with the remaining avocado slices and roe. Spread the remaining mousse over the top and fold the overhanging salmon strips over to completely enclose the mousse. Cover with the plastic wrap and chill until firm, about 3 hours. To unmold, place the mold in a bowl of warm water for 10 seconds. Unwrap the plastic from the bottom of the mold and invert the mold onto a serving plate. Gently shake to release the mold and unwrap completely. Serve with toasted croutons or crackers.