Process pears until smooth and press through a sieve or strainer to make very smooth. Chill thoroughly.
Chill simple syrup thoroughly. Simple syrup is equal parts sugar and water that is heated until sugar is completely dissolved. 1 cup sugar and 1 cup water will yield 1 1/2 cups simple syrup.
Put pears, simple syrup, salt, and vanilla and lemon juice (if using) into ice cream maker and chill according to manufacturer’s directions.
Apples should be cooked.