Step 1: Rinse the raspberries thoroughly in cold water and drain any excess liquid. If you prefer a seedless jam, mash the berries with a potato masher and pass them through a fine-mesh sieve into a large, heavy-bottomed pot.
Step 2: Add the granulated sugar and freshly squeezed lemon juice to the pot with the raspberries. Stir the mixture well to ensure the sugar begins to dissolve.
Step 3: Place the pot over medium-high heat and bring the mixture to a full rolling boil, stirring constantly with a wooden spoon to prevent the bottom from scorching or burning.
Step 4: Continue to boil the mixture for 15-20 minutes, or until it reaches 220°F (104°C). Perform the cold plate test: place a small amount of jam on a chilled plate; if it wrinkles when touched, it is ready.
Step 5: Skim off any excess foam from the surface. Carefully ladle the hot jam into sterilized glass jars, leaving about 1/4 inch of headspace. Seal the lids tightly and allow them to cool completely at room temperature.