Cucumber lime jalapeno granita for oysters

Bahan-bahan (12 porsi)

• 1/2 cup cucumber (, peeled, watery seeds removed, diced into ~1cm/1/2" cubes)
• 1 tsp lime zest
• 3 tbsp lime juice
• 2 tsp caster sugar
• 1 tbsp finely chopped jalapeño (, deseeded (Note 1))
• 12 - 24 oysters ((Note 2))
• Jalapeño slices (, for decoration (optional))