In a stainless steel mixing bowl, whisk together the eggs, 1/2 cup of the sugar and a pinch of the salt. Whisk in the champagne. Place the bowl over a pot of simmering water. (The pot should be sitting over, not in the water.) Whisk vigorously until thick, about 3 minutes. Remove the bowl from the pot and place it in a bowl of ice water. Cool until room temperature, stirring occasionally. Whip the cream until soft peaks form. Fold the cream into the cooled champagne mixture.
Spray a loaf pan with nonstick spray. (I like to use a pan approximately 3 1/2 by 11 1/2 by 3 1/2 inches but a 9-inch square pan will work.) Line the pan with plastic wrap, making sure to press the plastic wrap into the corners of the pan. (The nonstick spray helps the plastic wrap stick to the pan. The plastic wrap makes the terrine easier to unmold.) Gently spread half of the champagne cream into the loaf pan. Place the pan in the freezer for at least 1 hour. Refrigerate the remaining champagne cream.
Combine the water and the remaining 3/4 cup sugar in a small sauce pot. Bring the sugar and water to a boil over high heat and boil for 1 minute until the sugar is dissolved. Cool the mixture to at least room temperature. (It can be made several days in advance and refrigerated.)
Peel and pit the 2 pounds of peaches. Puree them in a food processor. Strain the puree through a medium holed sieve. There should be 3 cups of peach puree. Stir 1/2 cup of the sugar syrup into the peach puree. Add the remaining pinch salt and the lemon juice. Taste the peach puree. If it is not sweet enough add a little more sugar syrup. (The amount of sugar necessary will depend on the natural sweetness of the peaches. Extra sugar syrup can be used to sweeten ice tea.) Freeze the peach puree in an ice cream machine according to manufacturer's instructions.
When the peach puree is frozen, spread it evenly into the loaf pan over the frozen champagne cream. Let the puree become firm again in the freezer, then carefully spread the remaining champagne cream on top. Freeze the semifreddo for several hours until hard. It can be made several days in advance.
Peel the remaining peaches. Cut them in half and remove the pits. Cut in 1/2 inch pieces. Taste the peaches. If they are not very sweet, stir in a little granulated sugar. Often peaches are naturally sweet enough and do not need additional sugar.
To unmold, invert the loaf pan onto a cutting board. Run a wet hot towel over the pan. Remove the pan. Carefully peel off the plastic wrap.
Slice the semifreddo and place on a plate. Serve with some of the fresh peaches.