Heat olive oil in a large saucepan over moderate heat.
Simultaneously prepare your pasta in a separate pot as per the package instructions.
Add the garlic and onions into the saucepan; simmer until fragrant and onions are slightly translucent, approx. 3-5 minutes. Stir often to avoid browning.
Add the drained tuna, diced tomatoes, and Dijon. Hit it with some kosher salt and ground black pepper, and mix well.
Allow to simmer, stirring often, for approx. 5 minutes or until mixture thickens a bit.
Add the spinach and the frozen peas; stir to mix.
Once the pasta is ready and drained, add it to the saucepan. Mix well until all of the noodles are coated.
Serve right away.