In a medium saucepan over medium heat, bring water, milk, and salt to a simmer. Gradually whisk in cornmeal, making sure no clumps form. Cook for about 5 minutes, stirring occasionally, until cornmeal is cooked. Remove from heat, pour cornmeal into a 10 inch round pan and smooth out the surface with a rubber spatula. Let polenta cool to room temperature, then transfer to refrigerator for at least 2 hours.
In the meantime, mix together tomatoes, red onion, basil, and capers. In a small bowl, whisk together olive oil and vinegar. Pour over tomato mixture and toss to coat. Salt and pepper to taste and set aside.
Prepare grill or grill pan on stove top. Cut polenta into 12 even wedges and brush both sides with olive oil. Grill for 3-5 minutes per side, until polenta is golden brown.
On a large serving platter, scatter some arugula. Add grilled polenta wedges, then top with tomato mixture and burrata. Drizzle balsamic glace over dish. Serve immediately.