Preheat the oven to 350°F, with a rack in the center position. Line a baking sheet with parchment.

Tumble 2 cups of the blueberries onto the parchment and sprinkle evenly with the sugar. (A good bit of the sugar may fall onto the paper; carry on.) Roast for 18 to 20 minutes, until the skins burst and thick, purple juice pools on the parchment. Carefully lift the long edges of the parchment toward the center and tip the berries and their juices into a medium bowl. (If you wait too long, the juices will harden.)

Stir in the remaining ½ cup fresh berries and the lemon juice. Let cool. (To speed cooling, stir over an ice bath).

For each serving, swirl as much of the blueberry compote as you like through yogurt. If desired, top with one or more of the suggested add-ins. Refrigerate, covered, for up to 2 weeks, giving a stir every now and again.