Pre-heat oven to 400f degrees. Chop potatoes and radishes into similar bite-sized pieces. Coat with olive oil in a bowl by hand using 1 tbsp of oil, salt and pepper.
Line a baking sheet with parchment paper and spread the potatoes and radishes across in a single layer. Bake for approximately 30 minutes until browned (toss around once mid-way through baking to get all sides).
A few minutes before removing radishes and potatoes, start working on the shallots. Thinly slice shallots and add to the 4 tsp of oil in a small pan on your stove, saute over medium-low heat until slightly brown (don't burn!). Add chopped rosemary to the oil and turn off heat.
When potatoes and radishes are ready, place back into the bowl and add the shallots, oil and rosemary on top and toss with two spoons to coat the potatoes and radishes.