Season the chicken breast with half of the sea salt and black pepper, then grill over medium heat for 6-7 minutes per side until fully cooked.
Bring a small pot of water to a boil, carefully lower the egg into the water, and simmer for 7 minutes for a jammy yolk, then immediately transfer to an ice bath.
In a small bowl, whisk together the extra virgin olive oil, lemon juice, Dijon mustard, minced shallot, and the remaining salt and pepper to create the vinaigrette.
Place the fresh baby spinach in a large bowl and toss with half of the prepared vinaigrette until the leaves are lightly coated.
Slice the grilled chicken into strips and peel the cooled egg, cutting it in half lengthwise.
Arrange the sliced chicken, halved egg, sliced cucumber, and halved cherry tomatoes over the bed of spinach.
Drizzle the remaining vinaigrette over the top and serve immediately.