Step 1

Toss the quartered or sliced berries in a Dutch oven or similar sized pot with 1/4 cup (50g) of the sugar. Let marinate for a few hours, stirring every once in a while, or overnight.

Step 2

Add the plums to the pot and cook over medium heat, covered, until the plums are cooked through. Use an immersion blender to puree the berries and plums, leaving them chunky, if you wish.

Step 3

Add the rest of the sugar (4 cups/800g) to the pot, and attach a candy thermometer to the side, if you wish. Put a small plate in the freezer. Cook the jam over medium-high heat, stirring in the sugar, until it dissolves. Continue to cook, skimming any scum that rises to the surface during the first part of the cooking. Stir it frequently with a silicone spatula or flat-bottom utensil (so you can scrape the bottom as you stir) until the jam has thickened and the jam sticks in a clump to the bottom of the spatula or utensil. (As shown in the post.)

Step 4

When you think the jam is done (even if using a candy thermometer), test it by turning the heat off and putting a teaspoon-sized dab of the jam on the chilled plate and return it to the freezer for a few minutes. When you nudge it with your finger, if it wrinkles or appears jelled, it’s done. If not, continue cooking it a few more minutes, stirring frequently, then test it again. When ready, add a bit of kirsch or lemon juice, if using, then ladle the jam into clean jars.