Soften the cream cheese. Leave it at room temperature for 20–30 minutes so it blends smoothly without lumps.
Make the quick berry sauce. In a small saucepan, add 1 cup berries, 2–3 tablespoons water, and 1–2 teaspoons sugar substitute. Simmer for 5–7 minutes, stirring, until the berries break down.
Lightly mash, then cool.
Blend the base. In a mixing bowl, beat 6 ounces softened cream cheese until fluffy. Add 1 cup Greek yogurt, 2–3 tablespoons sugar substitute (to taste), 1 teaspoon vanilla, a pinch of salt, and lemon zest if using. Mix until smooth and creamy.
Taste and adjust. Sweetness can vary by sweetener.
Add a little more if needed. The mixture should taste lightly sweet and tangy.
Chill briefly. Spoon the mousse into small bowls or jars. Chill for 20–30 minutes to set and let flavors meld.
Top and serve. Spoon cooled berry sauce over each portion.
Add fresh berries on top for texture if you like.