Step 1: Approach by the cell pattern. Sheets of monotonous tumour cells with pale or clear cytoplasm, grouped in nests and divided by fibrous septa packed with lymphocytes, is the defining microscopic appearance of a testicular seminoma.
Step 2: Place its origin. Seminoma develops from the germ cells lining the seminiferous tubules and is the most common pure germ cell tumour of the testis, occasionally appearing in extragonadal midline locations such as the mediastinum.
Step 3: Use prognosis as a confirmatory cue. The very high cure and survival rate, its radiosensitivity, and preserved fertility all describe seminoma, supporting the identification.
Step 4: Rule out the alternatives. Teratoma would display organised tissue from ectoderm, mesoderm, and endoderm, not uniform lymphocyte-rich sheets. Non-seminoma and a generic differentiated germ cell tumour are umbrella terms that do not match a single clean histological picture of clear cells with lymphocytic stroma.
\[\boxed{\text{Seminoma}}\]