Grilled Chicken Panzanella with Cucumber and Feta

For the chicken: 

Juice of 1 lemon

canola oil

kosher salt

pepper

garlic, minced or pressed

chicken breasts, butterflied into halves (4 pieces total)

For the salad:

unsalted butter, at room temperature

garlic, minced or pressed

Salt and pepper,

1-inch thick slices sourdough bread (about 4-5 cups when cubed)

large or 2 small cucumbers, sliced and quartered into wedges

vine-ripened tomatoes, diced

crumbled feta cheese