Wash 1-2 cups of mixed berries thoroughly under cold water. Remove stems and hull the strawberries if using. Dry gently.

In a medium saucepan over medium-high heat, add prepared mixed berries, 1/4-1/2 cup sugar, and 1-2 tablespoons lemon juice. Stir gently.

Bring the mixture to a gentle simmer for 8-10 minutes. Stir occasionally, allowing the berries to break down and soften.

Transfer the berry mixture to a blender or food processor and blend on high until smooth.

Pour the blended mixture through a fine-mesh sieve into a mixing bowl to remove seeds and solids.

Let the coulis cool to room temperature, then transfer to an airtight container. Store in the fridge for up to 2 weeks.