Place a small plate in the freezer before beginning. The chilled surface will help show whether the syrup has thickened sufficiently.

Peel, core, and cut the pears into approximately 1/2-inch (1.25 cm) pieces. Measure about 6 cups (900 g) of prepared fruit.

Combine the pears, granulated sugar, lemon juice, lemon zest, and salt in a large bowl. Stir until the fruit is evenly coated, then let the mixture stand for 30 minutes. The sugar will become wet and syrupy as it draws juice from the pears.

Scrape the pear mixture and all accumulated syrup into a wide 4-quart saucepan. Set the pan over medium heat.

Cook for 5 to 7 minutes, stirring frequently, until the sugar dissolves and the mixture begins to simmer. Scrape along the bottom and corners of the pan so no sugar crystals remain.

Reduce the heat to medium-low and maintain a steady simmer for 25 to 35 minutes. Stir more frequently as the syrup thickens to prevent scorching. The pears should become translucent and tender while some pieces remain visible.

For smaller fruit pieces, press the mixture two or three times with a potato masher. Do not mash it completely; pear preserves should retain a chunky texture.

The preserves are nearly ready when the bubbles become slower and glossier and a spoon drawn across the pan leaves a path that closes gradually. If using a thermometer, the mixture should approach 218°F to 220°F (103°C to 104°C) at sea level.

Place 1 teaspoon of the hot preserves on the chilled plate and return it to the freezer for 1 minute. Push the edge with a fingertip. The syrup should wrinkle slightly and move slowly rather than running like juice.

If the sample is runny, continue simmering for 3 to 5 minutes and test again. If the preserves become unusually thick in the pan, stir in 1 tablespoon (15 ml) of hot water.

Remove the saucepan from the heat: Stir in the optional vanilla extract. The preserves should look glossy, with tender pear pieces evenly distributed through the syrup.

Carefully spoon the hot preserves into three clean half-pint jars or airtight heatproof containers, leaving about 1/2 inch (1.25 cm) of space at the top.

Let the containers cool uncovered at room temperature for about 1 hour. Cover and refrigerate for at least another hour before serving. Do not leave the preserves at room temperature for more than 2 hours.

Stir the chilled preserves before serving: Spoon them over toast, biscuits, oatmeal, yogurt, pancakes, or soft cheese.