Microgreens are usually grown on non-edible substrates, whereas edible substrates enable a fully edible, ready-to-eat product. This study evaluated mustard (white, red and green) microgreens grown on an edible gellan gum substrate within a “farm on the fork” system. Agronomical parameters (dry weight, germination, cotyledon area) and bioactive properties (phenolics, antioxidant capacity, phytomelatonin) were assessed during germination. Essential oil supplementation was also evaluated for sensory and antifungal purposes. Gellan gum at 20 g/L was optimal, supporting high germination (red: 95.7%, green: 98.3%, white: 100% at 72 h) and growth without irrigation. After 9 days, white mustard showed the highest fresh weight (63.1 mg/seedling), hypocotyl length (3.52 cm) and cotyledon area (43.5 mm²), while red mustard had the greatest nutraceutical value, with the highest carotenoids (76.4 µg/g FW), flavonoids (4.56 mg/g FW), antioxidant capacity (9.02 µmol TE/g FW) and phytomelatonin (25.5 ng/g FW). Essential oils did not affect biometric traits or antioxidant profile at harvest, although transient rises in flavonoids (+0.34 mg/g FW), antioxidant capacity (+0.97 µmol TE/g FW) and phytomelatonin (two-fold) occurred at early stages (day 3–6). Overall, gellan gum—alone or with essential oils—enabled safe, effective production of ready-to-eat mustard microgreens without compromising growth or nutritional quality.