In a bowl, toss the tuna steaks with lemon juice, two tablespoons olive oil, soy sauce, mustard, garlic powder, salt, and pepper. Set aside to marinate at room temperature for 30 minutes.

In a large frying pan, heat remaining one tablespoon olive oil over medium-high heat. Add tuna (discard the marinade) and sear undisturbed for one and a half to two minutes per side, according to your preference. Transfer to a clean cutting board and let rest five minutes before slicing against the grain.

In a bowl, stir together the cucumbers, watermelon radishes, rice vinegar, soy sauce, sesame oil, chili garlic sauce, maple syrup, salt, garlic powder, and sesame seeds.

Divide the rice between four bowls. Top with a few slices of tuna, cucumber salad, carrots, avocado, and scallions. Garnish with sesame seeds and serve immediately.