Dissolve the cornstarch: In a large saucepan, whisk the whole milk and cornstarch until completely smooth and no lumps remain.
Cook the flan mixture: Add the coconut milk, sweetened condensed milk, and salt. Cook, over medium to medium-low heat, stirring constantly, for 12–18 minutes, until the mixture is thick, glossy, and bubbling, with a pudding-like consistency.
Lightly rinse a 10-cup mold, bundt pan, or pudding mold with water, then pour out the excess, leaving behind a very thin film of moisture. Carefully pour the hot flan mixture into the mold, then gently tap it on the counter a few times to release any air bubbles.
Let the flan cool at room temperature for about 15–20 minutes, then cover with plastic wrap (pressing it gently against the surface of the flan) and refrigerate until fully set, at least 6-8 hours, but preferably overnight.
Make the berry sauce: In a small saucepan, combine the mixed berries, sugar, lime or lemon juice, and a pinch of salt. Cook over medium heat, stirring occasionally, until the berries soften and release their juices.
Pour the berry mixture through a fine-mesh strainer, pressing with a spoon or spatula to extract as much sauce as possible. Discard the seeds and solids. Let the sauce cool completely before serving.
Unmold the flan: Once fully chilled, place a serving platter over the mold and confidently invert it. Gently lift the mold to release the flan. If needed, briefly wrap the outside of the mold with a warm towel to help loosen it.
Spoon some of the cooled berry sauce over the top, letting it gently cascade down the sides. If desired, decorate with fresh berries and grated coconut for extra color and a festive presentation. Serve chilled, with the remaining berry sauce on the side for spooning over individual slices.