Cut radishes in half lengthwise and cut each half into 1/4" thick half moon slices. Melt butter in a small skillet, add radishes, and cook over medium low until tender, 3 to 4 minutes. Stir in salt and lemon zest, taste and adjust salt level as needed.
Top toast with a smear of fresh ricotta, a sprinkle of cooked radishes, and finish with parsley.
Notes
I love rye bread but it can often be overpowering for the delicate ricotta and radishes. Use wheat if you want a nice balance (or a lighter rye!)