Leave root and 1-inch stem on beets; scrub with a brush. Place beets in a saucepan; cover with water. Bring to a boil. Reduce heat; simmer 1 hour or until tender. Drain. Cool beets slightly. Trim off beet roots and stems; rub off skins. Chop beets.
While beets cook, place each chicken breast half between 2 sheets of heavy-duty plastic wrap; pound to 1/2-inch thickness using a small heavy skillet. Combine yogurt and Sriracha in a heavy-duty zip-top plastic bag. Add chicken to bag; seal. Marinate in refrigerator 30 minutes, turning occasionally. Remove chicken from bag; discard marinade.
Preheat grill to medium-high heat.
Combine 2 tablespoons oil, 1/2 teaspoon pepper, 1/4 teaspoon salt, and next 3 ingredients (through shallot) in a small bowl; stir with a whisk.
Place chicken on grill rack; grill 6 minutes on each side or until done. Cut chicken into 1-inch strips.
Brush remaining 1 tablespoon oil over corn; sprinkle with remaining 1/2 teaspoon pepper and 1/4 teaspoon salt. Grill 10 minutes or until lightly charred, turning corn occasionally. Cool. Cut kernels from ears of corn.
Place 1 cup spinach on each of 6 plates; top each serving with 3 ounces chicken, 1/3 cup beets, 1/3 cup corn, 1 cheese slice, and 1 tablespoon pecans. Drizzle 1 tablespoon dressing over each salad.