Preheat oven to 350F with rack in center position.
Heat 2 Tbsp olive oil in a 4.5-quart Dutch oven over medium heat.
Add 4 oz diced pancetta and 1 diced onion, sauté for 9 minutes until pancetta renders and both brown.
Add 3 minced garlic cloves and cook 1 minute more.
Add 1 1/2 cups dry Arborio rice and toast for 4 minutes, stirring to coat in fat until rice smells nutty and looks slightly translucent at edges.
Pour in 4 cups chicken broth and 1/2 cup water, stir well.
Cover with tight-fitting lid and transfer to oven.
Bake for 35 minutes without opening lid.
While baking, rinse 3/4 cup frozen peas under cool water until thawed.
Remove Dutch oven from oven and stir in 3/4 cup shredded Pecorino Romano, thawed peas, and 1 Tbsp butter.
Taste and add salt as needed, keeping in mind pancetta and cheese are already salty.
If too thick, stir in more broth or water to reach desired consistency.
Finish with balsamic vinegar, lemon juice or zest, and fresh herbs if desired.