Place the 1-inch beet cubes in a clean glass fermentation vessel sized to the batch.
Add exactly one chosen starter: homemade whey OR additive-free sauerkraut brine.
Add the filtered water and set a clean fermentation weight over the beet so every solid remains below the liquid.
Fit an airlock or another self-venting, pressure-aware closure. Ferment at room temperature out of direct sunlight.
Ferment for 2 to 8 days depending on room temperature. Refrigeration readiness is a deep ruby color and pleasantly tart flavor, not a taut lid or fixed gas schedule.
Remove the weight and strain the kvass into a fresh glass jar, leaving the beet behind. Add the measured raspberries directly to the strained kvass.
Cover, refrigerate, and use within 1 week. Strain out the raspberries when serving if desired.