Add green and black cardamom pods, whole cloves, broken cinnamon stick, black peppercorns, and fennel seeds to a dry skillet over medium heat and toast for 2 to 3 minutes, stirring occasionally until fragrant.

Pour 1 cup of water into a small saucepan and bring it to a gentle simmer over medium heat, avoiding a full rolling boil.

Transfer the toasted spices and sliced ginger along with a pinch of nutmeg into the simmering water and let the mixture simmer for 2 to 3 minutes to infuse.

Add 1 cup of sugar to the spiced water and stir constantly for 1 to 2 minutes until the sugar dissolves completely and the syrup becomes clear.

If using, add dried rose petals to the syrup, steep for 30 seconds, then remove the pan from heat immediately.

Allow the syrup to cool to room temperature for about 10 to 15 minutes, or speed up cooling by placing the pan in an ice bath.

Strain the cooled syrup through a fine mesh strainer or cheesecloth into a clean glass bottle, pressing gently on the solids to extract all liquid, then discard the solids.

To serve, fill a tall glass with ice, pour in 1/4 cup of chilled masala syrup and 1 tablespoon of fresh lemon or lime juice, top with 3/4 cup chilled soda water, stir gently, and garnish with fresh mint leaves if desired.